Surface tracking ground - lock

I am testing surface tracking mode indoor without gps using rangefinder. it is partilay working as I expect. many time when moving the switch to track GROUND it wont engage. I wont to make it more robust.
I guess the problem lies in glitches of the sensor I am trying to follow it by the log. I guess it timesout and reset the target value. I can see that under AP_SurfaceDistance.cpp there is “SURF” message. the logger is enabled and as I understand this message is not part of the bitmask mechanism. I am not able to see the message. can someone help me understand why is it not recoreded to the log ?