PCR tests have a low false positive rate, however, there are 3 possibilities all quite possible: (1) She was just infected a few days ago and is pre-symptomatic, (2) She was infected about 10-14 days ago, remained asymptomatic, but may still be contagious for a few more days, or (3) She was infected a couple weeks ago, is no longer contagious, but is still shedding dead virus which is detected by the PCR test.Not sure whether this is true positive or false positive, Now, she has to self quarantined for 10 days.
Hopefully another test can be done ASAP.
I hope it was one of the latter two, but unfortunately it may occur that she is shedding dead virus for an extended period of time and can not get a negative PCR test for several weeks, I believe in some cases even months.