[Christ]

Rust. It's there. Trust me.

Check the corners of the floor pan, all the way around, under the car. Check the area around the panhard bar mount (the thing that connects near the middle over the rear axle, bolted to the body at one end, and the axle at the other).

Check for a wet carpet anywhere in the car, mostly behind the seats and at the extreme corners of the car. This is often indicative of plugged/cracked/broken/disconnected drain tubes from the hatch area of sunroof area (Si models).

Other than that, check for the obvious things that you'd look for in any car.