so why didnt it?

theres so much more to the events of the eleventh hour

just what is the significance of the door being closed whilst the doctor was there, yet open when amelia was gathering her stuff together?

maybe prisoner zero has used amelias identity in the 12 years the doctor was away

would explain the distinct lack of memory (no knowledge of daleks, etc)

The door was closed when the Doctor came out of Amelia's room, open a crack when Amy picks things up from the other room, and open fully when she left again. The significance of this is that Zero is already hiding out there - remember he had already escaped before the Doctor got there and the Doctor had already felt the presence of the filter.

The implication of the shadow passing across the window while Amelia is waiting is that it's Zero (at this stage we don't know what Zero looks like, and since it opened the door it could be running around the house), but I agree with others that it's more likely to be something else.

yeah i got that... i just wonder why it came out at that point

after all, when the doc is around is probably the worst time to reveal yourself
however, the doc had left at that point

The door was closed when the Doctor was there - it only opened after he left.

Emergency back-up plan I'd suggest. If it had killed Amy when she entered the room it'd have lost the psychic link it had been building up over the years and had no fall-back position.

Yeah. It was lying low, it didn't want to be seen, and didn't want anyone to know it was there. It would only kill if there was a threat to its cover, i.e. when Amy had seen it. Luckily she took the Doctor's advice in time, which is why it took human form and followed her.