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 could 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.
Here are some facts about when Stryd advertises its Bluetooth and ANT signals:
Stryd always advertises itself through Bluetooth. This is regardless of motion and whether or not Stryd is on the charger.
When Stryd has not yet been paired through ANT with a Garmin watch, and you attempt to pair Stryd through ANT, you must move Stryd a bit so that it starts advertising itself via ANT. The watch can then find Stryd and create a pairing.
If the Garmin watch is paired to Stryd through ANT, Stryd will activate itself as soon as you start recording a run on your watch (no movement required). It will flash when you start the watch.