Ah, the magic of Rendezvous.

I think what you're seeing is a result of the built in Rendezvous technology which allows for sharing with other computers and peripherals with no configuration required on your part. Someone in the immediate area also is running iTunes and is allowing you to access her iTunes library or some part of it. You can turn this sharing feature off in the iTunes preferences, under the iTunes name in the top menubar.
You might find Kate's bookmarks pop up in Safari and her iPhoto library might pop up in iPhoto. Heck, you might even be able to use her printer or digital camera if it's connected!

You can also turn off sharing in the System Preferences, located under the Apple menu at the top left.