Observed to be spotted (3)
I believe the answer is:
saw
I believe this is a double definition.
'observed' is the first definition.
(seen)
'be spotted' is the second definition.
(spot can mean to see)
'to' is the link.
(Other definitions for saw that I've seen before include "Cutting blade" , "Observed; tool" , "Viewed; maxim" , "Wood-cutting tool" , "Cutting tool or maxim" .)