Heathrow 1960's
My father managed to get airside at LHR, in his car, one foggy day by driving between Queens and T2. (???). down service road, was airside for about 20 mins, on taxiways.
Eventually asked someone where was A4...???????? Reroute was ICAO Compliant.
In 1970's I was often airside delivering in a non airside specific transit van.
glf