I was flying way up in the hills and I got a "loose or missing prop" warning message. My controller would not respond to my stick movements but the return to home function did work.

https://www.youtube.com/watch?v=Vmnip2etaHU&t=9s

I don't think we have ever been able to determine what this warning means. If you lose a prop, a Phantom will descend uncontrollably and crash, it cannot cope with only having 3 props. If the prop is loose, it most likely will fall off, same scenario. It is a mystery what triggers this message, but as far as I recall, no-one has lost an aircraft after receiving that warning. (Please chime in if you have lost an aircraft due to this warning)

Glad you got it home - - -  Bit scary all the same

Count your lucky stars you got a warning and not the real McCoy.

I've only had that problem with a NON-DJI battery running the .30 firmware.  With .20, I never saw this message, and sometimes with a non-dji battery, it would give me the message about it being a non-dji battery and not flying at all.