https://en.wikipedia.org/wiki/Vehicle_registration_plates_of_the_United_Kingdom,_Crown_dependencies_and_overseas_territories#Standard_plates Might be a Guernsey plate.
My neighbours are away too tilly2...didn't tell us where they were going either but left us loads of cat food in case their moggie turns up. Methinks cat didn't want to go to cattery and knew they were going away so legged it, lol!
It would not be very neighbourly of them not to tell you if they were allowing strangers to stay in their absence. Why not go round on some false pretext; "has any mail been left for me here?" and check it out.