My wifes computer had some issues and I decided to do a clean reinstall. Disconnected all drives and installed a new drive for windows. Not even formatted before windows install. So no remnants of old installs could interfere.
Reinstalled Windows 10 and Office 2016. Next I wanted to type in the activation keys. Only to find out that both were already activated. They must have called home and some Microsoft server somehow recognized the computer and reactivated the software. Convenient in this case but a little creepy.