We are trying to sell the house, nobody lives there, when people go in to take a look at it, sometimes they smell gas in the workshop (room next to cellar - where furnace is located). I have had two plumbers come out and spray water/soap, another with blow torch and finally called the gas company - NOTHING!
Gas man said he saw a reading when the furnace kicked on (rollback?) but the reading drop quickly. He said old furnace needs cleaning. Plumber came - cleaned out furnace. No smell until two days later, went back into the house and the workshop smelled again.
The workshop has two small holes to the cellar (where pipes go through walls), but the cellar doesn't smell.
The workshop has a gas line that then feeds into garage to go to kitchen upstairs to stove.
The workshop has a sewer pipe.

Help!

You're checking on the gas lines for the smell. Could it be sewer gas from a trap that's dried up due to non use? Back to you, Tom

Great call Tom. If the house has been vacant for some time, I would suggest dumping water in every plumbing fixture and any other drains (floor drains, washing machine standpipes etc). I can't count how many times Ive been called for a gas smell and it was a sewer gas smell.

One way to determine is to shut the gas off to the house at the gas meter. If your location is really cold, make sure to put some electric heaters in there to keep the place from freezing up. If you still get the smell, you will know it is not from the gas or furnace.