It seems to be related to that moment when the printer restarts. It does this when detecting an active serial connection, reboot, then accept serial input. It reboots just after "Recv: start, and then gets that “0.00000” info.
Heres the log from when I click “test connection”:
2016-03-04 12:34:58,558 - SERIAL - DEBUG - Changing monitoring state from 'Offline' to 'Opening serial port' 2016-03-04 12:34:58,572 - SERIAL - DEBUG - Connecting to: /dev/ttyUSB0 2016-03-04 12:34:58,581 - SERIAL - DEBUG - Connected to: Serial<id=0x1433bf0, open=True>(port='/dev/ttyUSB0', baudrate=115200, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitor 2016-03-04 12:34:58,583 - SERIAL - DEBUG - Changing monitoring state from 'Opening serial port' to 'Connecting' 2016-03-04 12:35:08,596 - SERIAL - DEBUG - Send: M105 2016-03-04 12:35:18,608 - SERIAL - DEBUG - Send: M105 2016-03-04 12:35:28,621 - SERIAL - DEBUG - Send: M105 2016-03-04 12:35:38,633 - SERIAL - DEBUG - Send: M105 2016-03-04 12:35:38,639 - SERIAL - DEBUG - Recv: start 2016-03-04 12:35:43,450 - SERIAL - DEBUG - Recv: Info: 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 2016-03-04 12:35:43,454 - SERIAL - DEBUG - Changing monitoring state from 'Connecting' to 'Closed' 2016-03-04 12:35:43,455 - SERIAL - DEBUG - Connection closed, closing down monitor