snoop

Short definition from Wiktionary

Verb

1. 1
   To be devious and cunning so as not to be seen.
2. 2
   To secretly spy on or investigate, especially into the private personal life of others.
3. 3
   To steal.

Noun

1. 1
   The act of snooping.
2. 2
   One who snoops.
3. 3
   A private detective.