Definition of snoop:
(v) :
to be
devious and
cunning so as not to be seen; to secretly spy on or investigate, especially into the private personal life of others
"if I had not snooped on her, I wouldn't have found out that she lied about her degree."
(n) :
the act of snooping; one who snoops
"be careful what you say around Gene because he's the bosses' snoop."