Well, it's not the neighbor's water heater. They've checked, their closet is completely dry. The last thing the manager can think of it that it might still be coming from mine, from some sort of release valve at the top. I've put a bucket under the valve to see if it catches any water. I'm thinking it won't, because right under that valve is pretty dry.
I wonder if there's a blockage farther down the line in the drainage pipe, and it's backing up from there?