You can find pet urine stains with a black light. The room must be absolutely dark, and the light around 4-5 inches from the surface. Even old urine stains will glow pale yellow.
I use a 24″ fluorescent light fixture available at WalMart for around $11. Since I don’t like crawling around on my hands and knees for the optimal 4-5 inches, I duct-taped the fixture to the leading edge of a rolling magnet commonly used to pick up stray nails in a shop. I’m sure there are other substitutes. Just make sure you angle the light forward a little bit. Also, try to do it in complete dark.
Why do you care about this? It can indicate the presence of an illegal pet, if you suspect that. More importantly, it will identify the exact location of urine stains so you can enzyme-treat just that space, and not the entire dwelling. Enzymes work wonderfully, but they are extremely expensive.
Does anyone else have some property maintenance “hacks” they’d like to share?