I think it depends on what expert you talk to. For what the Lifeline distributor in Toronto told me, in person, read post #275 here:
In the end I chose another brand of AGMs, Odyssey, and had them installed under my bed, next to the charger and inverter. You can see my set up in post #649 here:
The man that sold me my Webasto diesel furnace, and who was such a sterling help to me during my remodel, sold and installed the batteries. I asked Randall whether they needed to be vented. He, too, said no.
Thatís from two experts, working with AGM batteries everyday.
I would say that the warning that appears in the Lifeline material above is the kind of boilerplate that every manufacture of pretty much everything has to use these days, no matter how remote the possibility of a problem.
On the other hand, safety codes are for our safety. There IS a marine world code stating that batteries of all types have to be vented.
I might just poke a little hole to the outside and put a small vent in the compartment next summer. But I donít think building a box, like for regular wet batteries, is needed.