1
general::
Phrase(s): spy (up)on someone or something
to watch someone or something to learn secret or concealed information. (Upon is formal and less commonly used than on.) • Are you spying upon me? • I wasn’t spying on you! I was just trying to see who you were.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Secretly or furtively observe someone or something, as in The children loved spying on the grownups, or The company sent him to spy on the competitor's sales force. [Early 1600s]
American Heritage Idioms