Suspicious Mac Added To Account But Doesn’t Appear On Device List

Hi i just got a message on my iPad and iPHone and it said:

“Device Added to Your Account

A Mac now has access to iMessage and FaceTime. If you don't recognise this device, you can remove it in Settings.”

I checked in settings and the only devices that are on the device list are my ipad, phone and Mac. (I have had the Mac all year and it has been signed in, so i am unsure if this is the cause of the message).

If my Mac isn’t the cause of this then does this means that someone has access to my imessage and i am unable to remove them?

Does anyone know if I’ve been hacked or is this just a glitch?

thank you.