In my profile I saw this:
Accounts (48) - 1 hidden account
as seen in this image:
So, what is this "hidden account"? And how do I see it?
A hidden account is one that is...well.. hidden from view. You can see this in the Edit Profile & Settings, then click on Hide Communities, there should be two lists - Visible Communities and Hidden Communities and options to hide and unhide communities.
To see your complete list, go to your network profile - and all memberships are shown.