That's always the answer

Luminol is a chemical that reacts with hemoglobin to glow where very small amounts of blood might have been. That's usually sprayed, for detecting much smaller concentrations than what would show up under a UV light. That might be what you're thinking of.

I know from personal experience that splooge does not fluoresce under uv. Pee does however.

And what about when they call your bluff and turn "semen inspection time" into a "guess where I came this time" game?

Then combine it with the "release numbered animals" and say there was n+1 to find when there's really only n.