As a regular commuter and Airbus pilot I do occasionally hear the PTU running as a passenger without any obvious cause. In the beginning I used to ask the flight crew what was going on, if they had any hydraulic problems or if they knew the PTU was running all the time, only to be met by blank stares from the pilots. Apparently there is no obvious indication in the cockpit that the PTU is running on these occasions.
So I have no answer to this question, I can just confirm that it DOES happen.
It usually happens after engine start and during taxi, it stops once airborne with the gear up, only to start again on approach when the gear is lowered. Or sometimes only during taxi or only on approach.
An Airbus mystery!