with match() you basically wait for a response that is ends with "\x06\x0d". If the device you're communicating with, doesen't send this response (f.e. doesent send anything or sends something else, like in the case 0x15) then you will get an empty string in return, because the matched text will not be found and the timeout of match will expire.

Try sending with s.send("\x02",250). This way the extender will not wait for a specific string. Hope this helps (I havent used async communication yet).