Unknown device signed in to my iMessage
An unknown device signed in to my iMessage somehow, but when I signed in to my Apple ID to change the password, it says no devices are signed in. How do I find out what device signed in and prevent them from accessing my account again?
I do not normally use this Apple ID. It is one I created, but I do not believe I have used on my personal devices. I received an email with the device name, but that does not appear anywhere else on my account.
iPhone 6s