I made the same request on my personal environment. I found out that my AWS EC2 machines (in the same AWS availability region and DC) are reporting the same info in the "isSiteOf" field:
Then my feeling is that the GEOLOC_SITE is a unique identifier for the Data Center (since you can also freely modify the hostname, so the IsSiteOf I guess it's also a way of uniquely identifying an host). When I take a look at my windows host (not an AWS EC2) this is what I see
And the geolocation of the data center is correctly reporting the area where I'm located (based on IP address):
Hope this sheds a light on what is the GEOLOC_SITE..if you need further information I guess we should involve DEV.