Would my setup (details inside) prevent Google from indexing certain forums?

[Kai Bond Sin]

Hello all.

I had a quick search around for someone with the same problem as me but they all seem to be a variant of it, so I decided it would be better to post a thread about it.

Here's the situation: My setup is version 4.1.5

We have 2 forums.
The first one is a public one which is read only for none members, and can be posted in by members.
Recently we've setup a 2nd forum and from the very beginning we have made it hidden to everyone unless they are part of XYZ Usergroup.

Based on the above setting, and that it's a hidden forum since the beginning of its time, would it still be possible for Google to find it and index it as part of its search results?

I am confident that I have bullet proofed it because:

1. Forum is unviewable if not logged in. Displays a login prompt
2. When logged in with a user not in XYZ Usergroup, a prompt says there is unsufficient privaleges
3. Only an admin can move a specific user into XZY Usergroup

Is there anything I might've missed that Google/any other crawler can get through?

Thanks in advance for taking the time to read this.