You're a plumber called by a customer to fix the drip in their kitchen faucet. No crime has been discovered as of yet.
But now the FBI tells you to look at all the plumbing fixtures in the house, to see if you can detect criminal activity in the home. You find the brick of cocaine in one of their toilet tanks, and when you are finished, you call your FBI "handler" (no other word for it, at this point), tell them what you have discovered, and watch your customer get hauled away an hour or so later.
If you are doing this at the request, behest or demand of the FBI, you are a de-facto agent of the government. Anything you saw or removed and handed over to the FBI is fruit of the poisonous tree (because, you know, no warrant) and any such evidence will be thrown out before you get to trial.