Oh dear I hadn’t thought of that.
Actually I had and it is set out in several previous threads. Demonreach is a Blacksite prison, it’s location is likely to to be highly restricted even among the Senior Council.
Beings who go to Demonreach tend to do so as a one way trip. Ask Ethnui. I doubt inmates get conjugal visits (Thomas may be a first, naturally)
Even Wardens may only have travelled there by the Way, indeed Kemmler most likely only ever did so. Unless he got a fix on its position via longitude and latitude, all that is known is that it is an island on a body of water. You can’t get a fix on it via magic. You would physically have to search for it, or in Harry’s case just stumble over it. When Kemmler got booted out as Warden it may be that Fistful of Warlocks was actually Kemmler trying to physically locate Demonreach in the real world. Which is difficult, even with 21st C tech.
Mab would have closed off the way to Kemmler when he got booted, so whilst Warden he was not able to reach the island. When he finally died it mean’t the Senior Council could appoint a new warden with the Gatekeeper shadowing the candidate through the Way. The Senior Council may not even have known the location if it was only known to the Gatekeeper. Thus Peabody couldn’t get it with his ink, as like Harry the Gatekeeper was only around rarely, and Fortier an old wizard was dead set about visiting the place, but he may have let slip to Peabody the Way was via Arctis Tor. “Demonreach? Why would I want to go to Arctis Tor?”
When one thinks of the Gatekeeper one thinks of the Outer Gates, but perhaps he was also the Gatekeeper for other things, such as the access for Demonreach. And who knows what other Gates?
You didn’t think I had just the one Conspiracy Theory did you?