I trained all me Springers to retrieve money. Seriously!
As pups, after they were retriever trained to bring back a ball, sit, heel and stay, I would put out a couple US bills while the pup was at stay in the other room. I would go back and send them to retrieve by the command, "fetch" and they would go out and bring back one of the bills.
As they grew up I would continue the training with pheasant wings, and sent and would occasionally toss out a couple dollars inside the house and they would bring them back. I noticed that the dogs would consistently bring back a larger denomination bill instead of a smaller one. I would have fun with friends having them put put two different denomination bills, with ought me touching them and would send them out to retrieve, "the big one" and the dogs would constantly return with the larger bill.
I made a few bucks on bets doing this and one day when speaking with a friend that was with the DEA, he said, "Of course! It is easy to explain. You trained your dogs to return with money and they would bring back the ones with a sent of Cocaine!"
I was aghast and asked how and why. He explained, "All used money is contaminated with cocaine. If you have over 3K in used bills in your possession, I could arrest you for possession. There is enough residue on 3K of used bills to detect possession of cocaine."
I enjoyed those dogs and leashed them when walking past a bank.