A pure guess, but it might just be an abbreviation for 'Patrol', or Patrol Over Water Land, or perhaps a patrol over locations coded or abbreviated, or perhaps 'Patrol (or PR) OWL', with 'owl' being a patrol area or specific location. Additionally, if it is a PR sortie, then 'PROWL' could mean just that - prowling around an area, with the enemy aircraft being a sighting.
Some of the entries in the copy I have of BoB Cross's log book have similar names/codes, for areas or locations in, for example, the Arakan.