Weird Yellow Gradient

I keep seeing this yellow gradient when I'm using my machine.

 

For the most part I see it popping up in browsers, but it's possible it's only appearing in Firefox.

 

Moving the mouse causes it to disappear, and since it's so random I'm often too slow to react and leave it on screen.

 

This time I caught it.

 

Anybody know what this is?

 

Searching for the phenomenon returns nothing...

Done