Three decades ago the A310 had cockpit access to the avionics compartment below. You could even open the hatch below that and climb down to the ramp on a small ladder. Sometimes mechanics would exit the aircraft that way at the gate before pushback.
Some overwater jumpseat riders on a full plane would take a walkaround O2 bottle and go down for a nap. If you needed some cushions, you could borrow some soft luggage from the forward hold which was accessible through a door.
One colleague inbound to FRA dozed off in the lower compartment. The relief pilot went down and woke him at top of descent but he fell asleep again. He woke up once more when the forward cargo door was opened on arrival and climbed down the loader to the ramp. A German airport security officer saw him exit and the fallout was predictable.
There have been many cases of baggage handlers falling asleep in the hold and taking an unscheduled trip to another city or even another country.
For example: