From there, jump onto the banister and then down onto the concrete wall
Walk along the wall and then jump onto the tin roof of the small office building
Walk over this roof until you get to the cable that connects this building with the neighbouring warehouse building
Carefully walk the cable
and from the cable jump onto the warehouse roof
Go to the open skylight, and jump through onto the rafters below
That credits you with the secret, but you still have to find the treasure item. It's in one of the crates - find the two gas cylinders and (from a safe distance) shoot them
Lastly, you can just use the normal green door to leave the warehouse.