There is a bug in the 1.4 patch that allows mercs to see and shoot through some of the buildings. Its most noticable on the pier level. The Mercs in the garage at the main camp can see and shoot you from inside.
The intention was to allow tents to be shot through in the multiplayer game, unfortunately non of the patch team, Crytek or UBI tested that sufficiently in the single player mode...
If you are just playing single player you should use patches 1.3, 1.31 and 1.33 (1.33 adds SM3.0 for ATI cards)