How can I identify an unknown device added to my Apple account?

I received this message on my desktop and I am not sure if there was an update or something?

It says Device Added to Your Account. An Iphone now has access to IMessage. If you don't recognize this device you can remove it in System Settings.

I don't know what device it is talking about etc. I did get messages on my two Ipads that they can now be

used to make calls. Was this just an update thing? Or someone hacking me?

[Re-Titled by Moderator]