I've been reading about saccadic masking. Fascinating subject. In a nutshell, it's the phenomenon where rapid eye movement, like a quick scan from left to right, can produce a blurred image on the retina which your brain then ignores. Could explain why someone could do a quick check to the right, then rapidly left, and still not see an otherwise obvious object.
https://en.wikipedia.org/wiki/Saccadic_masking