FM stands for Factory Mutual, another certification organization similar to UL. They apparently have a more narrow focus on product testing than UL, as you don't see their name on many products.

I concur with most of the others: a flashlight that's dive rated should be safe enough for most "casual" gas leak encounters. Most of those will have some sort of FM/UL approval anyway. The converse is not necessarily true; ie: you wouldn't take a Streamlight Vulcan diving. <img src="/images/graemlins/smile.gif" alt="" />