Spy

To gain sight of; to discover at a distance, or in a state of concealment; to espy; to see.

To discover by close search or examination.

To explore; to view; inspect; and examine secretly, as a country; -- usually with out.

To search narrowly; to scrutinize.

One who keeps a constant watch of the conduct of others.

A person sent secretly into an enemy's camp, territory, or fortifications, to inspect his works, ascertain his strength, movements, or designs, and to communicate such intelligence to the proper officer.

a secret watcher; someone who secretly watches other people; "my spies tell me that you had a good time last night"