According to Bonjour's "Frequently Asked Questions" page:
When I disconnect a device from a network, does it remain visible?
Yes, for a while. Eventually, the DNS record reaches its time-to-live interval and disappears. As an app developer, if you connect to a host using Bonjour and the connection fails, you can ask the Bonjour to reconfirm the record. This process is described further in NSNetServices and CFNetServices Programming Guide.