There is a hidden square area on one of the wood beams I believe that will reveal a hidden room with the explosives on a desk.
The square area to activate is tiny so look hard it is on the wood though I'm pretty sure. It is on the left hand side beam as well I think. About the size of a matchbook in proportion to your character.
Look at your map a lot as well, as it will reveal a lot of the so called hidden areas.
