I moved into a new place, the bathroom smelled like piss so I scrubbed every inch of it. Still smelled like piss. Then I got more aggressive and ripped up the toilet seat that I assumed wasn't liftable at first (since it was stuck pretty hard). Turns out it was just a regular toilet seat but with this dark red splatter under it, especially concentrated around the rim that the seat rests on. I cleaned that too, came off pretty easily. Now I'm waiting to see if the smell comes back..
I'm curious though, what could that red stuff have been? Didn't look like blood, it was more.. slimy looking. Sorta like if you were to squirt some ketchup on the wall in small speckles.