Look at the seat rails and seat mounting brackets. They're pretty much never galvanized, just painted a light matte black and work great as slow acting moisture detectors. Trim covers them up these days, but move the seat all the way forwards or back and you'll usually either see some of that metal or be able to get to a position where you can. They'll also rust over time due to long term high humidity from a leaking window or door seal, or shoddy crash repair, or a slow leak from the heater core, so even if it's not flood damage there might be other issues.