Apple ID Compromised

Click on this link "If you think your Apple ID has been compromised" --> https://support.apple.com/HT204145

'Checklist: If you want to see if anyone else has access to your device or accounts' click here --> https://support.apple.com/HT212021

Security and your Apple ID - https://support.apple.com/HT201303

Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." Better for directly reporting an issue.

I don't know what type of notification you are receiving. If it is a text message or email it could be phishing. Otherwise I know I have received on occasion unexpected notices like that. Maybe Apple reset something on its servers. I think I saw them when I turned on a device I hadn't used in a long time and maybe Apple had quasi forgotten about it. If you have followed the "Checklist" suggestions in my previous post and don't see anything fishy then I wouldn't worry about it.