The act of snooping

One who snoops

“Be careful what you say around Gene because he's the bosses' snoop.”

A private detective

“She hired a snoop to find out if her husband was having an affair.”

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.”