One of the more unique properties of AGM batteries is that their chemical process is recombinant. In conventional flooded batteries, hydrogen is pulled out of the electrolyte (which is mostly water). This breakdown is why flooded batteries occasionally need to be topped off with water and why the explosive hydrogen must be safely vented away. The AGM battery will recombine the hydrogen and oxygen to put water molecules back together inside the sealed system. The hydrogen that would otherwise be gassed out as an unwanted byproduct is effectively “recycled”. It’s a highly efficient process; over 99% of the hydrogen is recovered. This is how AGM batteries get away with never needing outside water added after they are sealed at the factory. They can literally make their own water.
AGM batteries have a safety valve that will release pressure if the battery becomes dangerously overheated but you may never get that far. They will generally show clear signs of their imminent death long before the valve gives way. If your battery feels excessively warm, develops a bulge on any side, or is making crackling/bubbling/ticking sounds, immediately disconnect it, let it stand for thirty minutes or so to stabilize, and discard it in an environmentally ethical manner. After you have cleared the trouble that caused the battery to fail (usually overcharging), go shopping for a new battery!