Hey Cerisier I signed up to PPrune (declaring my MN UK epaulettes) so I could ask the question about the power lines sourcing the transponder and ACARS outside of the cockpit - how many and hedging the possibility that a timed (non-explosive) cutting device could have been laid and primed when the A/C was on the ground. Anyway after accepting my registration and posting for moderation it didn't appear, after an hour checking for my post - I'm now missing a 'security code' and need to refer to admin.
The internals/cabin of an aircraft are just a whole load of lightweight panels (to make it look better for passengers and give places to put and stow things) covering a huge bunch of mechanical/hydraulic/electrical systems. (obviously I'm not a prof and hoping Goldie will be up soon). Lots of these panels give access to very relevant areas of the aircraft (and some are quite large, like the aircraft). Often, as the aircraft on the ground is prepared for flight, there are loads of people going on and off doing everything from the highly technical and specified to the mundane - cleaning the bogs (ends) for example from the inside and outside, catering, fixing something, counting something, checking something. The only personnel check that I know of (tho' out of the biz for years) is the Airport Pass (designated Airside/Apron). The guy or gal in charge of the depatch of the aircraft, who is more concerned about ensuring that minor fault is repaired and the correct fuel is loaded and the pilot's not pissed (sorry in darkest moments...that British thing) is not paying attention, nor required to as far as I know, to the number of cleaners,etc that have got on or off the aircraft, or if that quiet but highly respected Engineer X didn't stay on unnoticed as he's always in and out of those panels.