[Ygorl]

Who says nobody is?

Well, as the monk-man in CoW, I suppose it's safe to admit that nobody is.
Even with low-level patrolling (moderate PD or a small force of real troops) a force of 5-10 monks would be discovered in 1-2 turns. If someone noticed that they were finding monks in all their provinces every turn, they might get upset.

I think it would be about impossible to dominion-kill a human player without them knowing it was happening. I did take out Jurri (playing as BF Ulm) in one game (me as GE Arcoscephale) that way but I had the support of a massive army (first Ermorian, then Arcoscephalean) in his capital to keep him from patrolling my skeptics into oblivion. It was just easier and safer to do that versus sieging a well-defended fortress.