Thanks guys for the feedback. I opted, for now, to just ignore it and throw the sh!t in the trash. It wasn't a large quantity by any means. It was basically residue of the powder, some of which collected in the corner of the bag.
4) A fourth possibility is that there is someone else on the block w/the same last name who is more likely to a) use/cook a certain nasal powder; b) has some enemies.
It is of course (99.5% sure) some dummy thinking he's playing a funny joke.
I'd report it to police in the event Mr Dummy is doing it to other people in the neighborhood.
Being as his sense of humor is this low, I'd bet 50/50 he left incriminating fingerprints on the bag.
That only helps if his prints are on file. But again, chances are fair that someone this dumb has encountered police before.