<human>: Can you summarize the given text in a few sentences?

So, my friends and I were pulled over after pulling in to a park to try to smoke some weed. The cops ask us to step out and he searches our car and finds 1g in our possession. Then they see a strange white powder on the driver side floorboard and ask if we have cocaine or "bath salts". None of us have ever done cocaine and we didn't know what "bath salts" were. They did a swab test that would turn blue if it was positive for cocaine or "bath salts". It turned blue and cops were trying to force a confession. Eventually they let us go and my friend who had the weed, bong, and pipe got a ticket for possession of "drug paraphernalia". So, why was there a false positive? What could've triggered that swab to turn blue?
<bot>: Got pulled over, floorboard of car swabbed positive for coke or "bath salts". I've never even seen that stuff in person. Why?