Do the walkers have no memory? That first guy's wife seemed to clearly remember where she used to live and contrary to the logic set foward about all other walkers/biters/zombies she even knows to open the door. This is shown in the intro to every episode. This clearly indicates some level of memory retention. She was the only one who tries to get in. no other walkers attempt to open any doors.
At about minute 34:16 the boy sees her and he starts crying. His crying and the men talking causes her to come up to the door. There's also another "walker" (who heard the noise) walking up to the door. Her, and the other one, have followed the noise from the house, and maybe the smell too.
The second time she comes to the house was because of the gun shots fired by her husband.
Throughout the series they mention several times that the "walkers" keep no memory of their "living" lives. You can see this explanation with test subject 19 at CDC (S1E6 "TS-19") and the test ran by Milton Mamet on Michael Coleman in S3E7 "When the Dead Come Knocking".
As for the door knob... I don't know. I believe it was added just to improve the thriller effect.
They don't have any memory of the lives they had when they were alive. On the comic book,
When Tyreese's daughter dies, Rick tries to kill the zombie she's turned into. But Tyreese says 'Maybe we should talk to them, we haven't tried that', but