For some reason, from my own iTunes library on my Mac, I can see another computer's (running Windows 7) iTunes shared library, even though they are not sharing their library (via Preferences -> Sharing).
I myself have shared my own iTunes library, and they can see it just fine. But I should not be able to see the other person's library when they are not sharing it. I checked the preferences myself.
Also, my own iTunes is logged into my iTunes account, while the other computer is not logged in to any iTunes account.
Why is the other computer's iTunes library visible to my own iTunes?