Steve,
Yes, they caution about that on their web site, saying not to be smoking when you open the bag. I wouldn't be surprised if a vented gas cap would be enough to let fumes build up.
I "should" receive mine by this weekend, so I've done a thorough house-cleaning in there in preparation. I emptied my home-made mouse trap (see long lost thread) and there must have been over 30 of them in there! I have to glue more peanuts on the bottle. Some of the little pests were able to balance on it long enough to eat about half of them!
Verne