How visible is a SharePoint Site Collection to users without access?

I've received a request to create a new "secret" site in our SP Farm which will have a very restricted set of users, and should be entirely invisible to other users of our SP site. Ideally, nobody* should have any idea that the site exists.

I'm planning on creating this as a Site Collection, partially so that it won't exist as a sub-site in any other site's All Site Content. Also, I intend to exclude this SC from search indexing, and of course it's permissions will be restricted.

Is there any other way a normal (non admin) user could see 开发者_如何学Pythonthat a Site Collection exists if they don't have permission to it?

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The only thing I can think of is if the user guessed the URL. Then they would see the "Not Authorized" page and know something was there.