Please note that, as Andrea said, if you're signed in to one site, you're signed in to ALL sites. But. You only have admin access to ONE site :) Not all. Not even if you're an admin on the #1 (main) site. You are only master of the sites you are specifically made admin of.
ipstenu, I'm having a similar issue where an "Administrator" account (which is registered on the main site as a "subscriber") of a sub-site I created as a test is able to access the "wp-admin" page of the main site despite having Mingle installed and configured to redirect all members (including myself, unless I manually access it) of the main site to the "Activity" page. This worked for that subscriber account before using it to create a sub-site, but not anymore.
The account does carry the same permissions I assigned to the sub-site, so it doesn't exactly pose much of a threat in terms of settings or pages, however it does have access to the Posts/Add Post and Media/Add Media pages which leaves it open to possible spamming.
Now, this MIGHT be an issue with the Mingle plugin, but I have noticed similar incidents before installing it where pretty much any user with a site inside the "network" can access the backend of any other site despite having measures in place to redirect them elsewhere. With that said it could also be that WordPress is a little too open, in that it sees anyone with "Administrator" role on any sub-site as that of any site in the network... :-/
In any case, it would be useful to be able to define a custom role for the "Administrator" of a sub-site when they create one. Such a thing is easy to create, but not to implement - I've tried a hard-coded option a while back and even tried a custom plugin, but neither have yielded these results.
Any help anyone could give with this would be much appreciated :-)
LJ