If you’re interested, there’s a Python script here which will discover UPnP-capable devices on your network and display some information about them:
It could be helpful in determining what is responding, and how. For each device which responds, it will show the manufacturer and some additional info such as its location (URI to an XML file).
If you do decide to try it, you may want to increase the socket timeout in the function discover_pnp_locations. It’s currently set to 3 seconds; I’d try it at about 10:
sock.settimeout(3) changed to sock.settimeout(10)