There was a case in Garland, TX where a couple found a grocery bag w/
several hundred thousand $$$ in it, and turned it over to the gendarmes.
The police found traces of drugs on it, and refused to give the money to the couple after the notification period expired. Dunno if they ever got the $$$ or not.
They could probably find "traces" of drugs on just about any bill picked at random. Sounds like an excuse to me.