Hey Brian,
Sorry to hear about your loss. I just found this thread.
The very first thing I thought of when I read your first post was a power outage or pump malfunction.
I once came home to a pump failure. The lights were on in the tank, but the main pump was not running. The second I opened my door to walk into the house I knew something was wrong from the smell. That Ocean smell. The smell of dried out algae baking under the lights. None of my corals were exposed to the air, but the algae on the sides of the aquarium were. That's what caused the awful smell. And I'm sure that algae was dying. I was a little worried about it as I got the main pump going again, and it flushed the dying algae back into my system.
Maybe something like this happened in your tank. Possibly from raising and then lowering the water levels, you also exposed some algae that started to cook under the lights.
Or maybe a pump malfunctioned while you were gone, but then started back up again, but for a while, your water level was low, and caused the room to stink. Then, the pump started again, but the algae that was exposed was already dead, and fouled the water.
A lot of it is a guessing game at this point. The odd thing is having two other aquariums affected.
Any UV's on the three affected tanks? If it is a bacteria problem, that's the one piece of equipment I'd want on there.
Hope you find some answers.