The flap is locked when the car is locked. Adding a locking cap is a bit overkill IMO .Also inserting a syphon tube is not easy.
The flap may be a bit too flimsy to prevent a determined thief, but you would still have a damaged flap before they discovered your locking cap.
Years ago, when locking caps were a thing, it was not unknown for determined thieves to steal fuel by punching a hole in your fuel tank under the car.
Also an anecdote that may be urban myth. Years ago, if petrol theft became a real problem locally an owner would occasionally set up a booby trap using a false filler tube inserted in the proper filler tube, cans of petrol stored in a shed etc filled with petrol heavily dosed with sugar. The thief would seriously damage their own car/ motorbike, and maybe reveal themselves to be the culprit. , I really hope its true.
