Sounds like some sort of overcharging problem. Yes, batteries do emit a gas (a friend of mine pointed out on the top of the battery that the gas was hydrogen - READ 'hydrogen bomb'!!??) I like to connect a long hose from the vent hole in the battery (if it has a single one) and snake it safely through the body & frame and hang it out the bottom of the car to positively vent the battery. I don't know if this helps anything, it's just a theory of mine that if the combustible fumes are far away from any spark, the better (such as a spark that might occur when a battery terminal is loose or slightly corroded and you hit the key and an arc occurs). It is also important to keep the battery full of water to allow less space for gas to accumulate. Soooo... ...in my opinion, connect a good long vent hose and feel safer. Besides possibly averting a bad situation, (an exploding battery) it will save your battery tray, inner fender, etc. from rusting/rotting. First thing I would do is establish that the charging system is not overcharging as I suspect, and remedy that situation.
Does any body agree/disagree with this reasoning. Respond if you think this is bunk, I enjoy learning new things, especially when substantiated by fact, rather than theory; as I have presented above.