Garmin's GTU 10 is a great tool for finding lost things, provided that you had the foresight to GPS tag them in the first place.
Simply attach the unit to your outdoor pet, your backpack, or your child at the park. Using Garmin's Web app or the Garmin Tracker app for Android or iPhone, set virtual geofences to receive notifications when, for example, your pet leaves the yard. Then simply fire up the Garmin Tracker app again to locate the GTU 10 on a map. Tap "navigate" and the app will even guide you straight to your lost valuable.
At least, as closely as it can given the hardware's limitations. Just what are those limits?