My dad has a unique situation has anyone heard of this before?

My dad a radical prostectomy 5 years ago, it was successful and he did the follow up testing for the next 3 years with an undetectable PSA. Post surgery, numbers should be .01 and even an increase to .03 is something of note. BUT now the most recent PSA results showed 6.25, (he did take multiple PSA tests to make sure the number wasnt a false result) So he went in for a PET Scan that should have detected the cancer if it returned, but nothing showed up. So he went in for a second more specific PET scan and still the test showed nothing. Has anyone heard of PSA numbers being so high, yet no cancer showing on a PET scan? Even the doctor is confused and hasn’t seen this before.