Scams (e-mail, text messages, and phone calls) are getting very good at closely imitating true Apple communications. Always be cautious. Here are some guidelines:
Identifying legitimate emails from the iTunes Store - http://support.apple.com/HT201679
Recognize and avoid phishing messages, phony support calls, and other scams - https://support.apple.com/HT204759
If you are uncertain about a message and a resource provided in that message, do not click on any links. Try to independently verify the resource by going to a support page on apple.com and use that to contact Apple or to verify the service the message is telling you to use. In this case:
Checklist: If you want to see if anyone else has access to your device or accounts - Apple Support
I don't have a Watch. On some devices the notification appears with a map but the location is not always very accurate