Secret Places (4)
Press the mechanisms on the side of the firetruck (left side of the image) to open one of the building's windows. The window is near the backside of the firetruck.
There's a "wanted" poster near the mail doors (first class/overnight mail). Press on it to open it.
Go to the washroom. The second locker from the left has a secret door inside it. This leads to a secret room with some odd anti-canine posters. I suppose it would be logical for aliens to hate man's best friend.
This one is somewhat difficult to find. It's in the room where the mail conveyor belt begins. The secret place is on the right side of the image. You have to duck and move into the slightly darker part of the column. I included the security screen in the snapshot (left) to make the area easier to locate.