I have direct experience of AGM batteries failing, and regret I didn't photograph the removed battery. It was an Odyssey (that one was actually made by Hawker Siddley), and one of a domestic bank of three on my (then) newly purchased boat in 2005. Odyssey batteries are a definite premium range.
Ok, the guts of the story... The domestic bank was failing to hold charge, and I discovered one of them was warm. When removed the battery was bulging... Therefore it CANNOT possibly have been vented... The boat was marina based, and had been plugged into the mains, but only had a completely dumb Zig charger