Receiver xbee receiving data even if the Transmitter is not sending?
The reason that it is printing characters with no input is that the readBytesUntil function has a timeout. If no new characters are received for a certain amount of time, "Serail.setTimeout()" seconds, it will just continue with the code. the default timeout is 1 second.

The reason you are not receiving what you transmit (or at least a reason) is that the RX code does not read from an xbee, but reads from regular "serial".

I recommend you try something simple first to test the xbee's configuration, like having TX run this:

void loop(){
    xbee.println("Hello");
    delay(1000);
}


and RX run something to echo serial like this:

 void loop(){
     if(xbee.available()){
         Serial.print((char)xbee.read());
     }
}


And make sure you set up an xbee object an the receiving side first.