in informal usage, to look around or investigate something in a sneaking or secretive manner in order to find or learn something, especially about someone's private affairs;
e.g. They decided to snoop around to find out about his activities of that night.
(noun)
a person who secretly tries to obtain information about someone's personal affairs;
e.g. She is a snoop who loves to meddle in things that are none of her business.
(noun)
a secretive investigation or exploration of something;
e.g. He took a snoop around the house to see if he could find any clues.