I have been DAMP tested twice in the last 18 months. These were random tests, so it does happen.
I would like to make a comment regarding false positives. When I was being recruited for my current role, I had to attend a private clinic to provide a 'sample' for testing.
The nurse and I joked a little as I filled in the paperwork. The question arose: "do many pilots test positive"? Her response was that very few did and that those who fail DAMP testing tended to be cabin crew.
Upon testing my sample, the nurse's demeanor changed. She said that I had tested positive to opiate use. I denied that this could be the case. Her attitude was triumphant!: she had busted a pilot and she sneeringly informed me that the company would be notified immediately.
I continued to insist that the test result was wrong. She replied that the testing was very, very accurate and that the result was almost certainly conclusive.
At my insistance, I spoke to the Doctor in change of this clinic and asked for a re-test. He examined that test vial and the colouration had now changed from "positive" to "inconclusive". A new test was given to which I tested negative.