Just in case anyone was curious to have some inside info on this...
The place (Palais du Tokyo) has some of the most lax security I've ever seen.
They used to have a world-exclusive hotel 'pod' on the roof of the place, which we had the luck to grace a few times - never seen by security or cameras.
We also got into the basement of the place, exploring its abandoned 1920s theatres, cinema spaces and more - never once without any problems.
You could get into the building by so many vectors - one entire wing of the palace is totally empty, with broken and boarded windows on one side - and on the 'active' side - nothing but a tiny grill.
the more you know...