Staining Ink against thieves

[monotoned]

I've just moved recently to a student's apartment, and my food is getting constantly stolen from an unknown person. My friend told me about an ink that you can spray on your stuff that is invisible, but if someone touches it it will stain the person for about 1 week.
Me and some other people whose food is also getting stolen is thinking about buying the stuff, does anyone know what it's called?

[unstableasatable]

the stuff the police use i think is called black ink, but it only shows in ultra V.
i wud try posting in the chemistry section aswell coss there is stuff like iodene that stains your hands for a bit, and china ink, but i think that ones permenabts, best bet ask chemists tho i wud say.

also, suspect your friend