Spy Tracking Device is an app that turns your iPhone into a tracking beacon that can be monitored via Google Maps. It does its job, but it requires the user to hide his or her iPhone with the person being tracked -- in a backpack, under a car seat, wherever. If you're suspicious enough about a person that you need to secretly track them, do you really want to plant your phone on them and hope for the best?
A search for "spy" in the App Store yielded an application called "Spy Tracking Device" from Scoutic (once downloaded, the icon IDs itself simply with the name "Tracking"). Basically, it turns your iPhone into a tracker that reports its movements to Google (Nasdaq: GOOG) Maps. If you "wonder where they go all day ... or where they have been," then this app will satisfy your nosy little mind.