I saw this message, "Control Signal Lost", for the very first time today, and actually during my first Vision App Ground Station Flight Mission.  I was in a remote area, in a large open field, with no power lines or communication towers for miles.  I have around 60 flights on my P2V+.
I assume that the message is referring to a lost transmitter signal.  But I'm not sure.

The Mission had 7 waypoints at 184' altitude, and the farthest waypoint was 1600'.  In the return part of the Mission between waypoint 5 & 6 and around 500' to 600' feet out, I looked down at the smartphone and saw the message just for an instant and it disappeared.

Assuming this is a Vision App message and not specific to Ground Station, I just thought it odd that it appeared for the first time when using Ground Station, after so much previous flight time, no where near the maximum distance in the Mission, and in the wide open spaces.

It did not affect the Mission, which went on to completion.  Has anyone else seen this message, and in what conditions?

It means your remote control lost the signal with your Phantom. Normally, that would trigger RTH. It doesn't when running a ground station mission though.