... They know who is on the other side. They will take precautions. Meetings in a totally dark room keep Anduriel in the dark (sorry). So could using sign language ...
Totally bright & shadow-free is another option. Recall the description of Vadderung's offices.
But even if an "inner circle" or "senior council" can keep their own meetings secret (I think you're right, that it's very likely(*)) it's a VERY different matter to keep their
operations secret. As soon as they come out of their sanctum and begin interacting with their cats' paws and dupes... as soon as
things start happening... then they show up on the various "radars" of the various supernatural powers. And then the smart manipulators -- the Mabs and Anduriels -- will be able to infer a LOT from what they see them doing.
... Mab has agents, but doesn't have that supernatural intel ability that Andy does, or Ivy. Keep it small and quiet, you can hide from her ...
I am honestly unclear that we know (or rather: I suspect we do
not know) what sorts of "supernatural intel" may be available to the Queen of Air and Darkness. It's only a few books ago that we learned about Anduriel's shadow-spying ability. Given her wide-ranging powers and her proclivity for being a step ahead of everyone else, I (strongly) suspect that there's
something there, as-yet unrevealed.
But "small and quiet" doesn't make the big changes. You can
either hide from Mab,
or set ambitious plans in motion. Once you begin
acting on your ambitions, Mab
will notice you!
(*) OTOH, some of these secret info-gathering methods are not at all well-known. Luccio, captain of the WC Wardens, seems ignorant of Anduriel's abilities, for example; and I suspect
she'd know, if the
Senior Council knows (n.b. the SC keeps secrets from one another, per WoJ, so maybe an individual or two knows of Anduriel's power (or maybe not), but the WC (including the SC)
as organizations do not know).