If you completely lose your keys (eg left them at a Post Office counter), the App will take a GPS reading at the moment the App disconnects from the MyTag.
You can see this location on a map on the App, so you know where to start looking for your keys. On iOS swipe right on the main map view. On Android click on the last location on the main map.
Once you arrive at the location where you left your MyTag, the App should reconnect automatically with the lost MyTag, and you will be able to ring the MyTag to find it.