If you can't find your Withings Go, check when was the last time it synced with the Withings Health Mate™ app in My devices > Withings Go. This information is displayed at the bottom.
Your Withings Go was 10-15 m away from your phone at the time of the last sync. Depending on when that was, it might help you find it.
If you've looked everywhere and still can't find it, reach out to Customer Service by clicking the Contact us button at the bottom of the page.