Finder shows a (bonjour) device that's not on my network

Had a strange event today.

I have a private and a guest wifi network, separated (broadcast by different devices), both WPA2 protected, with good paswords.

Today, on my macbook pro, connected to the private wifi, in Finder, I saw under the shared devices a (bonjour?) computer of a guest that was connected to the guest network. Apart from the fact that I find it peculiar that I can see a computer that's not on my private network, I'm kind of worried about the security of my private network. My macbook plays Itunes music over the private wifi network, so probably has bonjour enabled. The guest had left when I noticed, so I cannot ask her for info.

Any advice is more than welcome, as are tips of how to (easily) know which bonjour devices are in the area, or on the network.

Thanks !