What is it?

What do I do about it?  

Rebooted and it went away

Yep, I used search here and got zero..  going flying, hope I'm not being stoopit!!!

I've only gotten the error once, and it was because I was holding the drone while it was trying to do its initialization thing (rather than setting it down after turning it on).  Haven't had the issue before or since.  Maybe it was totally random, or maybe it's because my hand happened to be covering the sensor when it was booting up?

I had that warning once in fog...

Sorry, double post.

Could be as simple as condensation on the lens of the sensor or spec of dirt, if the warning is gone away then the sensor is ok.

I cut short my flight as this happened just for the next leg of a planned route.

The drone did seemed to drift a bit in a hover.

Memorial Day BBQ sauce on the drone???

I've had it happen once. I moved to a sunnier take-off location and the alarm cleared.