I really, really doubt it's a gag. Too random that you might go into his toolbox, rather than your own, to find a tool. He probably just got sloppy and didn't protect his stash well enough.
I see why people say don't bring it up, but it is also a golden opportunity to get a new sister, especially since you said you get along very well with him already.
Why don't you invite him over for NFL championship Sunday, just you and him, buy a 12 pack-- no, better, a case of beer, and see if it comes up in conversation? Maybe you could break the ice by asking, jokingly, which of the QBs still standing would look the best in drag (my vote would be for Mark Sanchez, followed by Drew Brees, but that's beside the point).
Either way, good luck!
