I received a notification that FaceTime and iMessage being used on an Apple Watch but not mine what to do
How do I protect my watch and other devices
Apple Watch Series 4, watchOS 6
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Check your device list to see if there is a device signed in that you don't recognize. You can remove the device using the steps in the article below. As a precaution, change your password.
Apple ID device list -Check to see where you‘re signed in.
What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your Apple ID or password.