In a recent episode 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 the prey.  Yet this did not seem to happen in this case.

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.

Are there other precedents for ambush behavior in the Walking Dead universe?