Forgot to say, but we found a suspected leak from the ceiling a few weeks ago. There was a pool of water on 1 of the benches, under where all the vintage computers were stored. It all poured out of the Atari's base, when I tilted that, so I drained as much as I could, and left it on the floor, tilted against the bench.
So, please don't leave anything you care about in that area. There are some water marks where plasterboard sections meet, so that's where we concluded it had come from. It had been raining at the time, so maybe it's the roof?