In the days before security you could roam around just about anywhere, if you knew where you were going or at least if you looked as though you did.
1973-1979 I was working for BOAC/BA in Speedbird House and just behind the back entrance was a door in the blast fence which let on to the apron by the Wing Hangar (Tech Block B). Around the bottom of the hangar was our preferred canteen so you would often be ducking under the wings of the 707s and VC10s parked there, if it was raining you would take a shortcut through the hangar, taking care to avoid any puddles of Skydrol. No badges or IDs whatsoever.
Another time, myself and a colleague decided to lunch in the new-ish Tech Block C and walk back via the aprons. There were parked that day the very first of the 747-200s, just off delivery, and a Concorde. We strolled up the steps to the 747 and asked one of the folks starting work on the (empty) interior if we could have a look around. Having done that we thought – why not? – and went up iunto the Concorde. It was deserted, and for ten minutes we had the run of the aircraft, cabin and flight deck, to ourselves.