In regards to the reticles and different objects matching the icons on the sign, could shooting the object (bike, symbol on clock, etc) with the respective gun be a key?
Also I've noticed on YouTube videos that when the bomb shelter is shot with explosive rounds the outside of the shelter becomes dirty from the ash of the explosives but the door remains clean. This most likely means that the door is a separate object in the programming of the game, it's lazy programming but it supports the theory that there is a significance with the bomb shelter
