Tried this over the weekend Mavic Pro Platinum and controller, no phone, everything worked fine, that is until the Mavic battery reached 15%, I was hovering underneath some tree branches and above bitumen pavement at the time and it decided to switch to Return to Home mode. I heard some beeps and it started to climb. As I had no phone attached I was unsure of where Home location was and unable to confirm, I guess it was likely to be where I took off from, but had no way to check. I immediately attempted to land, but with full down on the left stick it just ceased to rise and stayed hovering, of course I was also worried that it would climb and run into overhanging branches, as soon as I started to release the down position it started to rise again. I considered an emergency stop but a fall from 3m onto pavment would not be beneficial so I abandoned that thought. I was able to hold it in a hover with full down, wondering what to do. I found that I could still manipulate its horizontal position with the right stick so I moved out from under the tree to a grassed area but remained concerned about where it would go if I let it continue its return to home. I held it in this position until eventually the battery was down to 10% and it auto landed without incident. Very nerve racking.
I might try again in an open field and attempt a return to home with plenty of battery to determine if the home position was indeed where I took off and of course perform an controlled landing prior to 15% remaining battery.
Hope this helps anyone else who tries this!