There are 4 secrets in this level.
Secret #1: Duke the firefighter
At the beginning of the level, before you enter the post office, there is a fire truck parked on the street. Approach it from its left hand side and press use at the water pump. There is a hidden switch that opens one of the windows in the building right next to the truck.
Go inside the room, watching out for the fires that are all around. You will find there protective boots and devastator weapon.
Secret #2: Postal rifle association
When you enter the change room inside the post office, there are four lockers. In one of them is blue keycard. The one left to it (second one from the left hand side when you stand in front of the lockers) has a different texture on the wall opposite to its door.
Open that wall and prepare for a fight with 1-2 alien beasts that are inside the room. When you deal with them, go inside and collect chaingun cannon, ammo for shotgun, pistol along with some clips and a small medkit hidden inside a ventilation shaft. There is also a portable medkit hidden behind a grating - blow them up to access it. This room also has a security monitor. An extra secret consists in shooting the dog poster opposite the wall with the security monitor to reveal a closet containing RPG ammo and atomic health.
Secret #3: Wanted
Inside the customer service room, there is a "wanted" poster picturing Unabomber. Open it and crawl inside the ventilation shaft. Remember that you can destroy the mailbags blocking your way. The only thing you will find inside this secret place is a box of pipebombs and also a protozoid slimmer towards the end of the shaft, if it has not crawled on you already earlier.
However, the ventilation shaft will lead you towards a one-side invisible wall with a view over the main post office room, making it a good spot for camping in deathmatches.
Secret #4: Secret ventilation shaft
Inside the first mail sorting room (after the blue keycard forcefield), there is a conveyor belt that goes up behind the laser turrets. You should notice the big pillar there, that has its center oddly shaded darker than the sides. Crouch in front of it and crawl into the darker part - you will find yourself inside a ventilation shaft.
There is a shrinker weapon inside. Also, there are switches in multiplayer that allow easier passage throughout the level via this ventilation shaft.
Easter eggs Edit
- The message "trab pu kcip" is a reference to a The Simpsons episode. The message, when read backwards in a mirror, reads "Pick up Bart". This message appeared in the episode Brother from the Same Planet, in which Milhouse writes it on a wall obliviously, which in turn was a reference to Stanley Kubrick's "The Shining".
- The picture for secret area 3 is actually the Unabomber's wanted picture.
- In Secret 3 there is a poster of "Postal Rifle Association", in contrast to the National Rifle Association. There is also some pictures of the mailman's worst enemy, and where to "aim" on it - the dog.
- The exterior of the Post Office at the start of the level is modelled after an actual Post Office located at 501 East Oates Rd., Garland, Texas.