Observer overlooked what another type of paper would catch? (1,3,2,3,4)
I believe the answer is:
a fly on the wall
'observer overlooked' is the definition.
(a fly on the wall is a hidden or unseen observer)
I don't understand the remainder of the clue.
(Other definitions for a fly on the wall that I've seen before include "What we'd often like to be" , "Enviable spying position" , "An unnoticed observer" .)