It is terrible to lose your Stryd, but there is some hope. Stryd will advertise itself through Bluetooth. You can use a Bluetooth finder app such as "Find My Device - Finder For Lost Bluetooth Devices". You should be able to pick up Stryd within 100 meters (without obstructions, less if it is in the woods, etc.).
If you were wearing a watch, and you can see where you lost your Stryd, you could use a Bluetooth finder app and try to locate it by returning to the point where you last had your Stryd.
Unlike some phones, Stryd does not have an app or technology to be pinpointed. We are also not able to track whether someone else paired with your Stryd.