I think of the criminal case years and years ago where someone was arrested with a large sum of cash, and they asserted it was drug money because cocaine was found on the bills.

The defending lawyer just took a random sample of money from circulation and found the same thing.

https://www.latimes.com/archives/la-xpm-1994-11-13-me-62172-...

and if you look further, even ATMs get in on the game, by spreading it on other bills that pass through the mechanisms:

https://www.snopes.com/fact-check/cocaine-on-money/

So by the same token, I wonder how many ways the samples can be contaminated, the obvious one being someone touches money first.