I operate a light aircraft from a former WW2 airfield that has been deemed by the authorities as being " high risk " for illegal activities, just the fact that the authorities have considered the possibility shows that they have an eye on the situation.
There is far more monitoring of these sites than is obvious to the casual observer.
Aircraft spotters are an unusually good source of information for the police and should not be underestimated, it should be remembered that it was the spotters who some time back alerted the police to an illegal entry to LHR......... not airport security.