I don't think it would be difficult at all to make an xbee scanner. Have a micro cycle through all the channels and look for serial chat, then see if it's ardupilot chat.
But i don't know how well it would work if the drone xbees were using encryption. And you'd need 2.4ghz S1, 2.4ghz S2, 900mhz to really cover the xbees.