I recently have changed my Apple ID password so I was logging in on my current devices (Mac Pro, iPhone, and iPad). I recently sold an old mid 2010 white MacBook to a store. Before doing so, I was sure to boot from the Snow Leopard CD, use the Disk Utility to erase the hard drive, and I even ran a clean install of the OS. After logging in on my iPhone, it showed me messages such as "Andrew's iPad is now available for iMessage". It showed me on stating that "Andrew's MacBook is now available for iMessage" which scared me a little since I sold that a while ago. I unregistered the MacBook on Support Profile. I'm about to change my Apple ID password again. Is it possible that the old MacBook has had access to my iMessages? What can I do to prevent that further if so. Thank you.