V on n
II.10 The 'spy' group
These verbs are concerned with secretly watching, listening to, or finding out about someone.
Sloan mingles with the crowd waiting to go inside and likes to eavesdrop on their conversations.
But they'll read your post, and listen in on your telephone calls.
They portrayed him as a temperamental tyrant who employed private detectives to snoop on adversaries.
The verb spy is sometimes followed by upon instead of on.
eavesdrop snoop spy listen in
They felt that they were being spied upon.