In Episode 2, how did Mark get up the stairs?

This question is spoilerific! Read at your own peril!

In the Walking Dead: Episode 2, it turns out the St. John family are cannibals and eat survivors, and Mark's legs are the main course of the meal. You see a legless Mark crawling around on the main floor of the house just after Lee tells everyone to stop eating. Later on Lee confronts the mother of the St. John family, who is holding a gun to Katjaa's head. She slowly backs up the stairs as you approach, and near the top of the stairs gets grabbed by a walker who is stuck at the top of the staircase.

I immediately assumed this walker was Mark because:

1. it was trapped, and all we could see was its arms, so its legs weren't functioning.
2. We'd seen Mark dying due to blood loss 10 minutes before, so it could be him.
3. How would a random walker get upstairs and trapped without anyone else noticing?

If this was legless Mark, why and how did he get up the stairs? No legs means no stairs. Did someone put him up there, or was this simply a stray walker who magically appeared upstairs?