why is My Apple Watch Vibrating Without a Notification?

[AnakinEmory]

Peculiar. Maybe it's a silent alarm or a reminder, those can sometimes vibrate without a screen notification.

[IanTheodore]

Verify your settings -> sounds & haptics. It might be Prominent haptic which is turned On.

[AnakinStephen]

Could be a glitch. Try restarting your watch, it often solves minor troubles like this.

[LucaWade]

It happened to m'once. It was the stand Reminder, notifying me to stand after currently being inactive.

[YahyaJadiel]

If none of those work, you may possibly want to contact Apple support. They were exactly helpful when I had an concern.

[JasonKoda]

Thanks everyone! I'll try your suggestions and see if anything operates.