The house Morgan and Duane was in was not theirs
I thought it was. If it wasn't, I sure don't have an explanation for the doorknob thing, but then I also don't know why the wife-zombie was following him around.
She lived there with them after the outbreak, & died there from the bite fever. Rick mentions that he knows whose house it is when he was 1st there
As for why she might keep returning or following them? This is what I call the teddy bear phenomena, because that is where we 1st saw the behavior. Episode one, scene one, a little girl is dragging her feet through a parking lot, & bends to pick up a teddy bear, prior to being called out to by Rick, who she then charges & gets shot by
What's a zombie need with a teddy bear? You can assume it was likely the little girl's teddy bear, before she died. When the zombie is not consumed by feeding frenzy, they exhibit a lot of peculiar behavior, like sitting in the church, as someone mentioned, or going to pick up what use to be their teddy bear
In Jenner's 2nd event scans from the CDC you can see very minimal impulses randomly firing out from the brain stem. Based loosely on this we can postulate that some are firing into the memory centers of the brain, bringing about these echoes of the person they once were, as Milton Mamet calls them from his research
The neural scans of Test Subject 19
The most horrifying thing about this show is that they toy with the notion that in some way, some of their humanity is still there, but only when there's nothing to feed on around