I have 2 6V AGM batteries in an inside cabinet (that I am sitting on right now). There is a 100A fuse right on the + terminal to prevent discharging at a fast enough rate to cause them to outgas. I am comfortable with this, and have been for almost 4 years.
When I was working, one of our products had a 7AH AGM battery inside a sealed enclosure containing the electronics that it powered, and the charge controller that charged it (it was solar powered). Around 1200 units shipped over 10 years, none had any problem related to the battery being in the sealed enclosure.