Secret one
First get inside the main enemy compound, from where you can climb up into the gatepost

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.
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