Microsoft Office keys are pulled from the registry locations below. If you've rescanned a machine with the Rescan Asset button and there is still an Office key listed on its asset page, there must still be a license key present somewhere in the registry keys below or sub-keys contained within them. For Office, we look for registry values called DigitalProductID or ProductKeys.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office (64-bit machines)