Great to see OP get more than reasonable compensation, well handled by the hotel.

By the way, I happened to have been in ATH too that "flooding night" Sa/Su, and despite having had a room with a large window front and a balcony, absolutely didn't noticed ANYTHING from the rain that night - in fact, when I got up around 9am, the balcony was dry and I just noticed a little bit of water on the chairs. So it's very believable that this was really an incident just happening to a very small part of the building the OP happened to be in, likewise a leak or something similar existing that went unnoticed so far.

It rained heavily during the afternoon (on Sunday) again, the streets felt like rivers. Should have made a video. It wasn't Indonesia-style flood, but certainly more than I've ever seen in Europe. You could have swum down that road easily..