There is a way to find out the bases they host from. It will read on the sessions description. They would be smart not to host on their addons, however, if they are not willing to put in at least something that can be seen by space up in simviation for the public, then they really must not want anyone to interact with them in the VR-World. They also don't seem willing enough to post their sessions with English letters, so everyone can get an idea of what they are hosting.
All factions should be open with their aircraft and scenery's, GPEO is. I don't see why they don't just post their links in the open. If I were them, I would do like GPEO, and create the ID thing where the GPS can find, far from the base, and have the actual location of the base hidden far from the actual ID marker. Their members would need to do visual landings, or have a separate scenery for their faction for all of that, but the airports should remain in the same location. It can be confusing, but is well worth keeping the secret. Even if your foe has the scenery, they will need to find it. It takes time, but may be worth it.
GPEO has many of these hidden bases, including naval units. Get the Komodo AB scenery, and try to find all of the ships, lol. The South Tip Scenery also has a hidden base, as well as a Carrier Task Force far out at sea, where only few members can find it, but it is hard even for them to find it. I still need to make a special scenery so we can spawn on the carrier, lol. That scenery is one I was still working on, but may leave it abandoned, or keep it for later usage.