Some AGM battery mfgs recommend the batteries be located in a vented space for the safety concern that smkettner related. Some AGM mfgs don't recommend a ventilated space since their products are used indoors, in X ray machines or telecom backup, for example.
I have three AGM's inside my trailer and don't worry at all about them venting. Any top-tier AGM should be good for indoor space. Heat isn't a concern, either. I have them installed against each other. I've never observed them getting anything but maybe slightly warm while charging.