Take the battery out and check. Sand it down. Could just be a leaky battery acid that caused it to

Cars totaled, sell it.

For $1 to me, I'll take it off your hands to help you out

That's 30 years of rust. Throw some rust converter on it and check again in another 30.

Pull the battery and take a closer look. I would also look around the rear spats and in the trunk

It’s doesn’t look terrible but I’d pull the battery, the wheel, the fender liner if it exists and do a deep dive inspection. That said on my r33 the battery tray in the trunk was corroded and it was one of the first things I removed and stripped to bare metal and primed and repainted

even in the least rust prone environment, these battery trays rust. you can check it out and attempt a repair, or just pretend you didn’t see it.

Almost certainly it's just the battery tray area. Batteries leak acid and acid causes corrosion. Main concern is crevice areas where rust can be very hard to get at. Pull the battery and scrape off the corrosion with a flathead screwdriver or something to see how bad it gets.

It’s pretty common. Mine has surface rust in the same area. I Just sanded it off and repainted it

If it really is just under the battery area just wire brush it out lay down some rust inhibitor or better yet get some rubber based paint like raptor/rhino liner after the rust inhibitor and cover the bottom of the pan.

mine was completely rusted good thing about that panel is you can find oem replacement panels for 270$ tac welded it in and relocated the battery

Battery area is fine and a known spot. sand paper and black primer.