This type of thing is why I do two things religiously every time before I drop off a boat into the water. I pump a couple of puffs on my auto-inflator to make sure it is connected, working, and putting air into my BC, and I take a few breaths in and out through my regulator. If my air were turned off, I would feel it in the couple of breaths after depressurizing the rig inflating the BC. I have caught at least once where I turned my tank on to check pressure, and a DM turned it off when they brought the rig to the back of the boat.