See other user pages, currently though here are some neat ways to stalk a wiki that cannot be seen.
Recent changes is often not a hidden page, and it reports how many pages exist on the wiki and what edits have been made (the content of the edit is secret though.
If you know the creator, you can track the number of 'hidden' edits to a wiki, and find out how many pages they have brought about.
In truth, you can hide the content of the wiki relatively easily (excepting page titles), but you cannot hide that there is a wiki, nor the size of the community involved, well at least under current security.
Unless they are editing under a different account than that which created the wiki (or somebody else picked up the task of editing that wiki). It's pretty hard to track a wiki then... as I found out when I was going through them all to document them and found a couple I don't have access to.
Eh, true that.