Tried to dig in a bit into the details of dbus and avahi to properly answer this, but not much success yet. My current guess would be, that when you set up to communicate through dbus you get_object and set up a dbus.Interface, thus it seems like a bidirectional connection - in which case dbus should know when that connections is cut (or maybe even dbus-python winds it down when stopped), and notify the other end (avahi), which can wind things down too… Either way, the way it works in practice seems to make sense for advertising services and winding them down, so maybe just have to keep the behaviour in mind without much worry about the implementation details? But would be good to find out more. 
That POS project sounds great! Would love to hear more as your project/product develops!