Or... is there something that REACTS to freon in an interesting way...?
Freon (chloro difloromethane) is pretty inert stuff, as far as I know. According to the MSDS (Dupont: http://www.dupont.com/msds/40_37_2008fr.html), it is stable but "Incompatible with alkali or alkaline earth metals--powdered Al, Zn, Be, etc." I'm not sure what "incompatible" means here. Finely powdered Al can be pretty reactive stuff, with many things besides freon. It's a pretty good organic solvent I would think, so you could easily find some dye which would disolve in it. If it disolves easily in gasoline (or tricholormethane, even better), it would probably also disolve easily in freon, so you could test some things. In fact, a smudge of light grease in the right shape might be able to indicate that freon has been used. Maybe we should just stick with email? Anyway, how often do you think this stuff is actually used in real life? Interesting to talk about, but hardly relevant to anything, I would think. Btw, are we sure that this spray is just plain old freon? I looked around and I couldn't see any hints about the composition of this spray.