I second this suggestion. For a while I didn't realise Rail Crossing had a medical trader, because I didn't realise there was a door there. And that one is one of the easier ones.
My boyfriend also has big troubles finding doors. He really likes the game, but he gets frustrated again and again by it, because he often can't find the doors, especially in areas that have more complex wall designs, but with damages. The current one is the institute of Tchort west wing - he was stuck because he couldn't find one of the doors.
This really is something that should be adressed in order to prevent headaches among players.