Marks one notices (5)
I believe the answer is:
spots
I believe this clue is a double definition.
'marks' is the first definition.
(spotting is a kind of marking)
'one notices' is the second definition.
(I've seen this in another clue)
(Other definitions for spots that I've seen before include "Acne" , "Blemishes" , "Picks out" , "Result of outbreak, possibly" , "Catches sight of" .)