Is there a tiny, low power chip that could detect a person's presence at 5' or closer?

Answer 1

Approach I would take is to start with an infared phototransistor w/ appropriate lens plugged into an opamp differentiator. With a properly-designed differentiator, the output would ignore gradual changes like day/night or bias change w/temperature, but respond to relatively sudden changes like a person walking into view. Device could be postage-stamp sized, depending on battery.

Answer 2

a motion detector only requires a photoresistor and a comparator chip as a minimum. the comparison and detection inputs both come from the photoresistor, except the standard side has a capacitor to slow its response to change. a practical detector is generally broadly directional.