I was guessing Milton from the beginning. He knew where they were and all. Also access out of town and a car is only available to certain people.
Over all this was a great episode. Very intense. Kept me on the edge of my seat like the mid season finale. I actually cared about Andrea for the first time. Kinda wondering how she didn't get bit during the scuffle in the woods.
Now here is my complaint. In the previous episode when Andrea visited the prison she walked there in a day. Now it took all day and night?
I'm guessing that perhaps the first time when Milton was with her he drove her halfway there or so. Then they captured the walker and she walked the remaining way. That's why Tyrese and Co were so close.
Also, in the warehouse when she opened the door to stairs the first time. Wouldn't the walkers have already moved in front of the door preventing her from opening it the second time?