The middle-aged golfers had finished their last single-malt whiskies late one night this July, and the bartenders were closing up.

Then a bus pulled up to the Trump Turnberry hotel on Scotland’s west coast with a load of new guests, several staff members said. The doormen, dressed in kilts with long feathers protruding from their berets, ushered in more than 50 uniformed US military service members.

After gawking at a fountain encircled by stone horses and classical statues, the troops piled their duffel bags around the table of orchids under the crystal chandeliers of the wood-panelled lobby, checked into their rooms and headed to the bar to begin ordering some whisky of their own.

Throughout President Donald Trump’s term, officials said this week, the US military has been paying his money-losing Scottish golf resort to provide five-star accommodations to US military flight crews and other personnel during refuelling stops on trips to and from Southeast Asia, the Middle East and other locations.

The chairman of the House Oversight committee has questioned if the spending at Turnberry is a violation of a constitutional prohibition on government payments to the president outside of his salary — a provision known as the emoluments clause. Other House Democrats have said they expect the matter will now figure into their investigation of a possible impeachment.

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But an examination of military layovers at Turnberry, including a two-day stay by a reporter at the resort, reveals a more complicated picture.

There is little evidence of a systematic scheme to enrich Mr Trump. But the military bookings at Turnberry are the latest in a series of episodes in which the president’s private businesses have intersected with his public position in ways that he can profit from.

The pattern also raises questions about how military officials failed to anticipate the questions that would accompany a large number of US military personnel marching into the opulent halls of one of the president’s golf resorts at public expense.

Mr Trump’s defenders note that US military jets have been stopping in the region long before his election. A decision by the Pentagon to have its flights stop more frequently at the local airport was made under the Obama administration.

The military said the vast majority of US military personnel who have passed through since 2016 have stayed at other area hotels, not Mr Trump’s. On Thursday, the Air Force said in a statement that it had found 659 instances when its flight crews stayed overnight in the area in the past four years.

Of those stays, the Air Force estimated that 6 per cent, or about 40 — far more than had previously been identified publicly — went to Mr Trump’s property. Trump Turnberry was closed for renovations from 2015 until mid-2016.

Those who did stay there paid a discounted rate of as little as $130 (£105) a night, compared with a typical price of about $380 (£306) a night.

“To me, it was honestly just a hotel, a place to sleep,” said Nathan Wendzel, 33, a helicopter pilot, who spent a night at the Trump Turnberry last September, along with about 35 other members of his Iowa National Guard unit, on their way back to the United States from a trip to Kosovo. “It is better than a tent with no air conditioning.”

Neither Trump’s company nor the US military has disclosed how much government money has been spent at the Trump resort. But a dozen Trump Turnberry staff members, all speaking on condition of anonymity for fear of reprisals, said the military stays have been a regular occurrence and, often, encompass surprisingly large groups.

The Trump Turnberry hotel and golf resort in Scotland hosted the US military 40 times in the last four years (AFP/Getty Images)

Buses like the one that arrived in July periodically turn up at midnight or 2am carrying dozens of soldiers or Marines, several hotel staff members said. Less expensive hotels, like a TraveLodge and a Premier Inn, are next to the airport — the Trump Turnberry resort is about a 40-minute drive.

The bookings for US military personnel staying at the Trump resort are made by employees of the Glasgow Prestwick Airport, which has an incentive to curry favour with Mr Trump. The airport has become economically reliant on the military-refuelling flights, creating at least the appearance of a motive to steer business to the US commander in chief.

Michael Matheson, Scottish transport minister, told the Scottish Parliament this week that the Turnberry is one of 13 hotels the airport uses and that “Turnberry is generally booked only if other hotels are unavailable or if customers specifically request it.”

But critics said that the military stays at Trump Turnberry still underscore recurring questions that have grown out of Trump’s singular decision to remain the owner of the business that bears his name while holding high office.

Long stays by lobbyists and foreign officials at the Trump International hotel in Washington, Vice President Mike Pence’s recent stay at a Trump resort in Ireland, and the president’s highly publicized outings to his own golf clubs have all raised similar issues.

At times Mr Trump has appeared to promote his hotels at the same moment that he denies steering them government money.

“NOTHING TO DO WITH ME,” Mr Trump tweeted this week in all capital letters about the military stays at his Turnberry resort, before adding in parenthesis, “They have good taste!”

Even some guests at Turnberry questioned the arrangement. “It is completely inappropriate,” said Bennett Rodick, a Chicago lawyer watching the sunset from the hotel lobby with his wife. “You don’t want him commingling his business interests with his government interests.”

The US military has been using Prestwick as a stopover since at least World War II, in part because of the extremely long runway the airport offers, and its reputation for being largely free of fog.

General Dwight Eisenhower sometimes landed there, and after the war Scotland gave him permanent use of an apartment in a medieval castle not far from Turnberry that he frequently visited. In March 1960, Elvis Presley, then wrapping up his military service, stopped at the airport for a few hours and was mobbed by his fans.

But the last decade brought trying times to the airport. The US military’s stopovers declined with the end of the war in Iraq and the pullback of troops from Afghanistan. And commercial air traffic shifted to the larger Glasgow International Airport. The situation got so desperate that in 2013 the Prestwick airport, deep in debt, was sold to the Scottish government for around $1.

Mr Trump entered the picture the next year, when he bought the Turnberry hotel and its storied golf course — it was one of the courses that had hosted the British Open — from a company owned by the emirate of Dubai, reportedly for about $55 million (£44.3m). Soon after he flew with great fanfare into Prestwick airport on his Boeing 757.

Donald Trump waves to screaming protesters at Turnberry in Scotland

He announced that his resort was forming a cross-promotion deal with the airport. To make Turnberry “the finest resort anywhere,” he told reporters in 2014, “we need an airport.”

There would be “people coming in from New York, high-level people from all over the place — a lot of private aircraft will be landing with groups and individuals and we expect to be using Prestwick quite a bit,” Mr Trump promised.

Trump executives also began negotiating with Prestwick airport officials to try to ensure that they would refer visiting aircrews to the hotel, emails first obtained by The Guardian in 2017 show. Details of the House Oversight Committee investigation into the military stays at the resort were reported last week by Politico.

But at least for Prestwick, almost none of Mr Trump’s predictions came true. The majority of the resort’s customers are affluent North Americans, along with a smaller number of Asians and others who come on package golf tours. Few arrive via Prestwick.

These days its cavernous main passenger terminal is often almost deserted. The only airline that still flies into Prestwick is the discount carrier Ryanair. Only a handful of its flights come in each day, mainly from relatively small European markets. No flights arrive from London, Dublin or North America.

“Years ago we had more flights and other airlines, but it is very quiet now,” said Margaret Vincent, 57, pulling down the gates Thursday afternoon to close the empty airport bookstore next to the empty cafe, empty bar and empty foreign currency exchange desk.

But on the opposite side of the airport, the US military has brought back at least some of the business.

The Defence Logistics Agency signed a formal refuelling and aviation services contract with Prestwick in August 2016, under President Barack Obama. The contract began being used in a major way the following year, after Mr Trump took office. Through June, federal contracting records show, it has made at least 925 fuel purchases at the airport, worth $17.3 million.

A US military aircraft — often a C-130 Hercules transport plane — lands or takes off almost every day, according to local air plane enthusiasts who wait by the airfield to watch them.

Trump has come under fire over claims his resort profited from US military visits to the Prestwick base in Scotland (Getty Images)

In part because of the refuelling agreement, the level of US military air traffic has surged during the Trump presidency. After 145 stopovers in 2016, there were 257 last year and 259 in the first eight months of this year, the Pentagon said.

The number of stopovers with overnight stays, entailing booking rooms at hotels, has climbed from 75 in 2016 to 208 last year and 220 this year through August, according to Defence Department figures.

Trump Turnberry can seem like incongruous housing for military personnel. The white walls and red roof of the main hotel stretch along a high ridge overlooking the rocky coast of the Irish Sea.

The windows look out over stone steps descending through rolling hills to the golf course, with the historic Turnberry castle and lighthouse in one direction and the surreal dome of the granite Ailsa Craig protruding from the sea in the other. Each night at sunset, a bagpiper — also in a kilt — parades past the lobby windows, right in front of the helicopter pad.

Several of the military visitors complained that the resort was not a particularly convenient place. It is far from any restaurants or even a pub. A burger costs almost $26 (£21) at current exchange rates, and blended whisky starts at nearly $10 a glass (£8). A day ticket for hotel guests to play on the signature golf course costs $495 (£400).

Mr Trump visited during the 2016 presidential campaign and his staff passed out baseball hats with the slogan “Make Turnberry Great Again.” Since his election, Mr Trump and his family have also brought additional federal spending to the resort. He stayed at the hotel and played a round of golf there in July 2018, accompanied by diplomats, advisers and his Secret Service detail.

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“I have arrived in Scotland and will be at Trump Turnberry for two days of meetings, calls and hopefully, some golf — my primary form of exercise!” Mr Trump wrote on Twitter during that visit. “The weather is beautiful, and this place is incredible!”

Eric Trump, who helps oversee its operations, visits frequently, along with his own Secret Service detail. Asked Thursday about the stays by military personnel, Eric Trump declined to comment but praised his family’s property.

Records show payments of at least $64,000 (£51,500) to Trump Turnberry by the State Department in the last two years, although part of that money might have been refunded, the records suggest, after the trips ended. The Trump Organisation said it refunds to the government any payments made to Turnberry for those visits by federal government employees after accounting for the resort’s costs, but declined to provide details.

Mr Trump, after he was elected, transferred ownership of his resorts, golf courses and other properties to a trust that is controlled by his sons and company executives. But he still benefits financially.

The resort lost $4.2 million (£3.38m) in 2017, according to an annual filing in Britain, continuing a string of losses reported since Trump bought it.

The club had revenue of $23.4 million (£18.9m) in 2018, according to a financial report filed in the United States, its best year since the Trumps’ ownership. The company has not filed the resort’s profit or loss statement for 2018.