I think i figured out the Easter Egg. LIsten to my steps and reasons and comment back.
You must then shoot the bike down from the ceiling, as the first picture on the bottom of the sign shows
Then you must shoot trinity ave, sign on opposite side of map its the 2nd symbol the trinity sign
I believe sometime after round 25 you must save 1 zombie, the population counter must be at 1 like the picture that is realeased with the map, the 4th symbol is a atom symbol which is also on dials for the counter, you can see through the sign on the right side to see it, and 25 is when rictofen takes control of zombies.
Then you open the back lawn door for the drop of a nuke the 5th symbol is a nuke.
Then maybe the shelter opens of something about the shelter happens.
I think it makes sense to an extent with what ive learned from mutliple videos and playing around with it but I was not able to do it online, or by myself, because its difficult to save 1 zombie. Tell me your thoughts.