In the Civilian Base 1 level, there is a room that cannot be accessed, but you can see it on the map when you look at the second large room; it is on the northern side. To get to it, you must first destroy all of the lights in the mission. They cannot be targetted, so you have to aim manually. After you've found and destroyed all of them [luckily, they have no effect on the actual lighting], go back to the northernmost big room on the map. The wall will still be there, but you can now go through it when Meteor Crashing. Inside is a picture of the developers.