Desperate for answers, the owner of a Calgary Dairy Queen is offering a free Blizzard every week for one year to whoever can pinpoint the source of a mystery smell that has been permeating the northeast eatery since it opened in 2015.

"It smells like natural gas, it smells like propane, rotten eggs," said Sujad Bandali, owner of a Dairy Queen in the 11100 block of 14th Street N.E.

"As soon as you walk in you get a whiff of it then it kind of disappears."

The smell was first noticed just a few hours before the store's grand opening three years ago.

"We went into panic mode because we thought there was a leak," said Bandali. "We had firefighters come in and ATCO come in a few hours before we were supposed to open for friends and family. They tested everything and everything seemed fine."

They then got the building's landlord involved in the search.

"They sent out several people, experts to do an analysis, they checked out everything and there's absolutely no leakage," he said. "One of the building maintenance [workers] thought it was coming from the drain, from the sewer, so they flushed it out completely and we still had the smell."

Bandali said they continue to flush the drains regularly to no avail, so he came up with the free Blizzard offer as a last resort.

"I'm desperate," he said. "I'm looking at customers feeling guilty."

This sign is posted to the counter at a Dairy Queen in northeast Calgary offering free Blizzards for a year to whoever can find the source of a mystery smell. (Russell Bowers/CBC)

ATCO crews have been called to investigate complaints of a natural gas leak at least twice, Ryan Germaine, vice-president of operations for the Calgary and Edmonton regions confirmed to CBC News, but both times crews came up empty-handed.

"In this case we didn't detect any [gas], zero per cent," he said. "We also, of course, are checking for carbon monoxide, that's just a standard check we do. We'll check all the appliances, any of the piping or appliances that are using natural gas, just to confirm they're working properly, that there aren't any little leaks … everything was fine."

A more extensive search was done the second time.

"In this particular case ... we checked around the building, inside, outside, and couldn't find any indication through our monitoring equipment of anything natural gas related."

The entrance to the Dairy Queen where the smell has been lingering since it opened more than three years ago. (Submitted by Sujad Bandali)

Adding to the mystery, said Bandali, is the fact that not everyone seems to be able to smell the offending odour.

"It depends on how sensitive the person's nose is," he said. "Half the staff can smell it, half of the staff can't smell it. A couple of months ago, a customer came in and she could smell it so strongly she asked for the manager. We've had customers walk in then walk out, it gets quite embarrassing."

He's also talked to surrounding businesses, who say they haven't experienced anything similar.

Some customers have suggested the smell could be from freon coming from the cake freezer, but that was ruled out by the building's landlord. (Submitted by Sujad Bandali)

The Blizzard offer has only been up for a few days, and already some customers are coming up with ideas.

"I've had a couple of people suggest to me it could be the freon from the cake display freezer, but nobody has really told me a concrete answer," said Bandali. "They're coming up with suggestions the landlord has addressed."

Germaine said it's not uncommon for ATCO crews to be called about a possible gas leak only to discover the source is something else.

"It could be sewer, it could be some other industrial process if you're in an industrial area," he said.

"Even high-traffic areas, different vehicles, especially larger trucks, some of those things can have a similar smell."

But this case, he says, "is a little bit more of a head-scratcher."

Because natural gas is colourless and odourless, an additive provides the unique smell to warn of danger, said Germaine.

"Kind of a rotten egg smell is typically how we would describe what that additive smells like," he said.

"Everyone has a slightly different nose, of course."

A senior technician will be sent out once again, said Germaine, to try and find a source once and for all.

"Our expertise is to eliminate the potential that it's gas," he said. "In terms of determining what else it is, that's not our expertise but I understand the frustration for the business owner."