This article explains how to ping a lost Apple Watch from an iPhone using an option in Control Center and the Find My app.

If you’ve misplaced your Apple Watch at home or at the office, you can use the Ping My Watch feature in Control Center to locate it. Your Apple Watch must be nearby, as this option only works if your iPhone and watch are within Bluetooth range (a few dozen feet) or connected to the same Wi-Fi network. If they’re not, skip to the next section.

To add Ping My Watch to Control Center, first tap Settings.

Scroll to the More Controls section and tap the + next to Ping My Watch.