That is much more likely than it sounds. Law enforcement in the United States are fond of a $2 drug test that reacts with practically anything. Allegely, it waa supposed to serve as a field test (with follow-up tests at a lab) but these days is used as probable cause in a baggie.
This test has been known to land gazed donut enjoyers in jail as well as someone transporting the cremated remains of a recently deceased loved one. It would not surprise me if a (willfully) ignorant police officer eager to ruin someone’s day woud use such a test on produce mushrooms to assert cause to arrest.
While I cannot say that is what happened here, I can say it’s not merely plausible in the States, but expected, especially if someone shows signs they don’t have a family lawyer.
You seem eager to give law enforcement the benefit of the doubt, which I assure you in the US, at from the precinct level though municipal department, county sheriff, state department and federal level they do not deserve.