In "Secrets", episode 6 of season 2 of The Walking Dead, Maggie and Glenn had been in the pharmacy for a few minutes. Maggie then walks to the back of the store and starts rummaging through the racks of medicines.
Suddenly, and without warning, a walker grabs her hand from the other side of the shelves.
In every other instance that I recall, the Walkers reacted immediately upon sensing living prey by moaning and walking/shuffling/crawling towards them. Yet this did not seem to happen in this case.
We've seen evidence that most Walkers can smell living humans from several feet away (identifying Glenn and Rick as living after rain started to wash away the stench of the Walker organs they covered themselves with), and there have been numerous examples of Walkers responding to sounds and visual stimulus (while they don't seem to be able to tell a human from a Walker on sight without other cues such as odor, countless Walkers have demonstrated the ability to perceive, and react to, motion at varying distances).
Setting aside dramatic license, was this Walker waiting quietly until Maggie got within reach? I suppose it is possible that it somehow didn't realize they were there until Maggie was in reach, but I didn't see any indication that the Walker's senses were in any way inhibited. There was no obvious sounds of moaning, or boxes and furniture being knocked over as it heading straight towards its prey as we might expect based upon behavior seen from other Walkers.
Are there precedents for ambush behavior in The Walking Dead universe? Or are there precedents that show that Walkers don't always behave the way I described, and the appearance of intentional ambush may be coincidental and simply normal Walker behavior?