Spies talk of “shadowing” to refer to trailing someone secretly. The section of “shadow” means remaining near the one you trail while unseen. Similar to a shadow; at all times with you but mostly not noticed.
In case you are shadowing together with fellow spies, you exchange so as to avoid provoking suspicion. The spy who is presently observing the suspect is referred to as the “eyes,” who is in most cases found out or “burned. “