Before sailing off on a seven-day cruise from Port Tampa Bay this month, a man left his car at a valet service. But that's not all he left behind.

According to a Tampa police report, 15 handguns were in a display case hidden under a black sheet in the backseat.

When the man got back, the guns were gone.

The thieves could have made off with even more weapons if they had wanted to: There were also 10 rifles in the car that weren't stolen.

Gun thefts are on the rise in Tampa Bay, especially guns stolen from vehicles. That figure has doubled in Tampa this year and spiked in St. Petersburg, too. Other police departments around the country are noticing the same trend, as gun sales rise sharply in many states.

Those stolen guns are likely to wind up in the wrong hands, sold on the streets for $100 to $150, said Kevin Richardson, a spokesman for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

"People leave their laptops in the car, their purses," he said. "The only difference is a firearm can be used to cause serious bodily injury to someone. A laptop can't."

In Tampa, 285 guns were reported stolen last year. This year, that number is 375, an increase of about 32 percent. In St. Petersburg, it's gone from 285 thefts to 312, a 9.5 percent rise.

Weapons stolen from vehicles is a big part of the problem. In 2014, 51 guns were stolen from cars in Tampa. This year: 105. That's a 106 percent jump. In St. Petersburg, the number has risen by about 45 percent, from 73 last year to 106.

Law enforcement officials say that all too often, poorly secured guns are being swiped from cars that are often left unlocked.

Richardson said the Port Tampa Bay theft in which a burglar made off with 15 used handguns is an extreme example. The man who kept 25 weapons in his vehicle could not be reached for comment by the Tampa Bay Times.

More often, police field reports of one or two guns nabbed out of a glove box or center console. Tampa police warned gun owners last week to not leave guns in their cars, especially not in plain sight or an obvious place. Police even specified that cars in valet lots are becoming targets.

They also told gun owners they should keep the serial numbers of their weapons readily available for police, should the guns be stolen.

Richardson said thieves often seek out unlocked cars, even if there are no valuables in view. He said people sometimes go to parking lots checking car doors to see if they're open.

In St. Petersburg, groups of teens sometimes fan out in the streets, pulling on car door handles and looking for easy marks, said St. Petersburg police spokesman Mike Puetz.

Tampa's data, which was taken from January through November of this year, show police are also recovering more stolen weapons. Tampa police have taken 84 illegal guns off the streets this year, compared with 66 last year.

That's because there are more guns out there now than there were just a year ago, authorities said. Richardson said, in part, the spike in thefts stems from the general increase of gun sales and ownership in the state.

An indicator of increased gun sales statewide came from Florida Department of Law Enforcement Commissioner Rick Swearingen this month. He told reporters the average daily requests for background checks required for gun sales are up 45 percent from this time last year. The rise began after the Dec. 2 terrorist attack in San Bernardino, Calif.

Connor Ganisin, 23, a firearms salesman at Bill Jackon's Shop for Adventure in Pinellas Park, said the gun section was packed in the days after the mass shooting. He estimated the store sold 50 to 60 guns in three days. He's been working there for about nine months and had never seen the store so crowded.

Ganisin grew up around guns and is well-schooled in gun safety. He got his concealed-carry permit this year and is taking an online class on gunsmithing he can learn how to properly repair and take firearms apart.

"I wouldn't really want to separate myself from my weapon," he said, saying he wouldn't be comfortable leaving his gun in a car.

The store also sells devices that could hamper gun thieves. Those include lockboxes small enough to be stowed under car seats that can be opened only using keys, keypads or fingerprint technology, he said. They range in price from about $199 to $269.

While police recommend not leaving a gun in a car at all, they say secure lockboxes are better than the glove box.

It's unusual, Puetz said, for a stolen gun not to end up in the hands of a criminal.

"One act of leaving a car unlocked may perpetuate a series of crimes," he said.

Times reporter Dan Sullivan contributed to this report. Contact Sara DiNatale at sdinatale@tampabay.com or (813) 226-3400. Follow @sara_dinatale.