Use Find My to search for a lost Apple Watch
Apple Watch owners have access to a very useful function of syncing it with the iPhone, with which you can find each of the devices separately if it is lost. These functions are combined in an application called Find My. The watch finder tool benefits are available to iOS 10 users with watches on watchOS 3 and above. Depending on your watchOS version, you may have some restrictions on utilizing the Find My Watch app functionality. To use it and find your lost Apple Watch, you need:
Set up Find My on your iPhone
In Settings, tap your name
Scroll to Find My
Switch on Find My iPhone
Switch on Enable Offline Finding
Then tap to switch on Send Last Location