Go through the level and collect the Remote Mines, as you will need them later. Continue until you reach a point where there are two yellow cases on the right being watched by two guards. Take out the guards, then toss a Remote Mine near the two cases. It will explode, opening a passage. Go right and collect the health, then go back to the entrance of the hidden passage and go left to collect the Automatic Shotgun. It is a very useful weapon because it kills regular guards in one hit and the yellow guards in two or three hits.