I'm not sure what exactly you mean by non commercial use (maybe you mean testing, maybe it's a euphemism for military use? Maybe you are leaving room for some other implied concept you don't want to specify, maybe it's just lost in translation). But for now I'll just assume you mean testing the hardware before the site goes "live".
Ground Stations get built and are in plain open sight during construction and use. Most anywhere that has enough people to need a ground station has someone willing to track the construction and share it online. Very similar in concept to finding Supercharger locations before they are turned on.
Some get found in FCC filings and similar from other countries. Generally someone will drive out, send a drone, fly by to confirm.
Starlink US gateways - Google My Maps is a crowdsourced map of ground stations in construction and in use (and contrary to the name has ground stations not in the US in addition to the named region).
Starlink Global Gateways & PoPs - Google My Maps is another crowdsourced map.
Discover our data center locations list google data centers and there is a mention of those locations either having or intended to have a ground stations for Starlink on the Wikipedia page for Starlink.
https://starlink.sx is a good site that shows active ground stations (though it is a little busy visually and might take some effort to find them).
none of which have any reference of a station at Diego Garcia.
searches of r/starlink and r/StarlinkEngineering/ have no posts about Diego Garcia
Is it possible they started setting one up there recently and we don't know about it? Totally possible. I just don't have any reason to think one has started construction there or that one will start construction there.
And I'm not sure I'm on the same page as you regarding the scenario it would be built for or the status you might think it is in.