Upon returning to Mars City, you will find a Z-Sec zombie in the first room. Take him out, and a person in the shaft above will lower a ladder. Climb up and go through the ceiling to the following rooms.
After going down, you will need to collect a keycard to go through the locked door.
Additional health is dispensed in the infirmary.
After the infirmary, you can reach Marine command.
When you reach marine command, you will activate a video transmission from Sergeant Kelly. After the video is complete, your PDA will have upgraded security clearance, allowing access to the breakroom.
You will want to stock up on supplies. The Mars City Armory is nearby, locked with the combination of 584. This can be found in a PDA located near the armory but behind a door; it can be accessed from the other side.
Proceed into the breakroom to the left. While fighting off enemy Z-Secs, work your way to the main elevator in the area; just as you exit the break room, you will encounter a security drone that will protect you for the rest of the map.
When you reach and enter the elevator (where the drone parks itself), you can exit the map at any time. If desired, you can collect one last PDA in the map by entering the wall on the right and climbing up the ladder.