A hot engine will heat the carb and fuel will evaporate out the float bowl vents to the atmosphere - in a closed shed the fumes may be detectable - as long as no leak was found, I would pass it off as that, but keep an eye on things - it might be a leaky petcock, fuel pump, fuel tank or line.
An extreme example of a possibility; (this happened to a friend of mine and he got all hyper about it) he had been plowing snow, got done and filled the fuel tank right to the brim (actually spilled a small amount of fuel putting the cap back on the tank - he rode it up and down the 200 foot drive way an couple times to 'evaporate' the spillage before putting it away in his heated garage. The next morning he went into the garage and smelled gas - he found a small puddle of fuel under the 4 wheeler - he immediately opened the garage doors and pushed the quad outside - left the doors open and the quad out for about 2 hours so 'all the gas would disperse and prevent his house from blowing up' - like I said he was hyper. He called me and I went over to investigate. We started the quad up and moved it back into the garage. I started checking it for leaks - found none, but there was about a half of a drop of fuel hanging on the fuel tank vent line. I figured it was just spillage (overflow) from moving the machine. I checked the fuel tank and it was about 1/2 inch below the rim of the opening. My friend said it should be full - he just filled it before putting it away. I said "Well there's the problem - you are not supposed to fill the tank that full - you are supposed to leave about 1/2 inch of air below the bottom of the filler. With the tank completely full, as the fuel warms in the garage, it expands and can be forced out of the tank vent line. While we were discussing things I noticed the fuel level had risen about 1/4 inch. I put the cap back on the tank and while we were having a cold one, i noticed fuel dripping on the floor. Sure enough it was coming out of the tank vent line. I removed the gas cap and lo and behold, the fuel was nearly to the top of the filler - we left the cap off and about 20 minutes later there were lots of drips of fuel and the garage was starting to smell of gas. The fuel was now overflowing out of the filler neck and running down the sides of the tank. I removed the fuel line from the petcock, attached a length of hose and drained about a pint of fuel out of the tank. Put it all back together and said bye. He called me the next day and said it stopped dripping, but the fuel in the tank was about a 1/2 inch higher than it was after I drained some out. I just said fuel expands as it gets warm - either keep it cold, don't put so much in it or use it for 15 to 20 minutes after filling it. He hasn't had the same problem since then.
