Yes, over the years this has been one of the most common questions we
get here:  The patient has a history of sinusitis and has symptoms of
sinusitis, and even other tests suggest infection--yet the CT scan shows
clear, even to a trained ENT.

The answer to this conundrum is that no test is 100% foolproof.  CT
scans have maybe a few percent rate of false negatives (CT scan clear
even though patient has sinusitis), and up to a 40% (!!!) rate of false
positives (CT scan shows abnormality even though patient feels fine).  I
was particularly unlucky.  I have had two sinus surgeries in which each
time, the CT scan taken prior to the surgery didn't show all the
problems the surgeon found after he operated on me.  The last time, the
CT scan showed some evidence of infection, but not the pus-filled
mucocele that the surgeon found during the operation.

But a good ENT shouldn't rely on tests alone.  With your history of
sinusitis (including two surgeries), and your symptoms, he ought to
strongly consider that you've got sinusitis again, no matter what the CT
scan shows.