Why does a MacBook Pro keep dialing in its location through Find My Mac after a clean install?

Unfortunately my iPhone was stolen yesterday. I did have Find My iPhone enabled, but shortly after the theft my iPhone stopped updating its location. I believe the device was turned off, reset, or had it's SIM card removed.

When finding my iPhone I did notice something interesting though, a MacBook Pro I used in the past but which was handed on to a colleague still shows up in my devices. It, however had a clean install of OS X after the HD was replaced, and is now only linked to my colleague's personal iCloud account, and doesn't even have Find My Mac currently enabled.

Therefor my question is why and how this MacBook Pro is still being tracked in my iCloud account after a complete reset/reinstall? Is it being tracked by MAC address whenever it comes online? Do I have a chance of locating my iPhone in a similar way later on?

I'd really like an authoritative answer on this, as this observed behavior raises additional questions on abuse of Find My Mac after buying a second-hand computer for example, as it seems you need to rely on the seller removing this device from his iCloud account.

Solution 1:

Here's what Apple says:

Because of how Find My Mac works, you must deselect the box next to Find My Mac in the OS X System Preferences > iCloud pane to remove your device from your iCloud Find My iPhone, iPad, iPod Touch, or Mac device list.

One of the ways this persistence is implemented is by writing a Find My Mac token to NVRAM, the persistent memory in Intel Macs. You can see this token by running the following command in a terminal:

nvram -p | grep fmm

On my Mac, the result includes my name, Apple ID, and the name I gave the computer, along with some encoded information that probably includes a key of some sort.