Was sent to spy out the enemy camp.
An example of spy is Mata Hari who was employed by Germany during the first world war.
Spied the ship on the horizon.
Spying into the neighbor's activities.
Spy plane, spy satellite.
During the Cold War, Russia and America would each spy on each other for recon.
I think I can spy that hot guy coming over here.
An example of to spy is hiding behind a bush and watching someone through their window.