What happens when you test positive for THC when on probation for a DUI?

There's a good chance that you're going to be found in violation of your probation.  If you think this was a false positive, you need to ask for a re-test, as soon as possible.

Possession of marijuana is against the law.  You're on probation, and a drug test has connected you with the illegal substance.  You could be served with a search warrant at any time, since the positive test is probably enough to satisfy the probable cause standard, which is lower because you're on probation.