After updating the Firmware on friday, I flew the Mavic on saturday and used the RTH feature with no problem.
Sunday am, taking off from exactly the same spot as he previous evening....after a few minutes, the Mavic became unresponsive, would not turn, change altitude or other. It would not respond to the controls. I activated the RTH, and while it was beeping....it kept flying away. Kept goimg and going.
Fell on a forest where we looked for it for hours to no avail. Its gone.
See attached photo.
Has this happen to others?
Why would it happen?
What is the best way to contact DJI?

Going from your screen-shot (as no other flight records supplied) I would have to guess that the wind blew it away.
At 427m the wind could well be much stronger than the slow RTH speed, from the photo the Mavic is facing home but if the distance was still increasing, then it was being blown away.
If you had control then switching into sport mode and dropping altitude would probably have got you back home.

Has this happen to others?
Yes, not understanding the change in wind speed at altitude has blown away quite a few aircraft.
Why would it happen?
Flying at high altitude and well out of sight greatly increase the risk of losing your aircraft.  
What is the best way to contact DJI?
http://www.dji.com/service/repair You should sync your flight records first.

Harbourside Posted at 2017-5-22 06:38
Going from your screen-shot (as no other flight records supplied) I would have to guess that the wind blew it away.
At 427m the wind could well be much stronger than the slow RTH speed, from the photo the Mavic is facing home but if the distance was still increasing, then it was being blown away.
If you had control then switching into sport mode and dropping altitude would probably have got you back home.

True. But the real question is... What RTH height was set. I don't think he has put 500m as RTH height.... Wouldn't the craft drop by its own to set altitude? Then I think we could speak of a softwareissue. Since he was still able to follow the drone. Unless that screenshot was the last communcation with the drone.

Nees Posted at 2017-5-22 07:04
True. But the real question is... What RTH height was set. I don't think he has put 500m as RTH height.... Wouldn't the craft drop by its own to set altitude? Then I think we could speak of a softwareissue. Since he was still able to follow the drone. Unless that screenshot was the last communcation with the drone.

If you are above your set RTH altitude it will stay there, not drop down. It only ascends to your set alt.

Have you tried Find My Drone?  That should walk you to the last recorded GPS location.  

sorry for your loss of the Mavic, here is the link to start the RMA process to see if anything can be done. They will review your flight to determine the cause.
As the prior post stated, I'm guessing you had a strong headwind and the Mavic could not make it back. It was in RTH mode, unless you had it set to hover and it didn't move towards home.
http://www.dji.com/service/repair

Jeff7577 Posted at 2017-5-22 07:18
If you are above your set RTH altitude it will stay there, not drop down. It only ascends to your set alt.

Right, I forgot.

Hmmmm not a lot of wind! Weird