I do this sometimes! It is usually a sign that someone is gassy... and sometimes a warnig that more is coming soon! Or maybe an old dirty diaper is in a trash bag that's not airtight or something. It's the sort of thing that I never would have thought about before having kids. But now I get all Sherlock Holmes when I smell poop.
Do you ever smell a dirty diaper when no diaper has actually been dirtied?