''Retired Mac repair person – doesn't mean I know everything, but that I've done a ***whole lot*** of repairs and have seen things.''
Your display has layers, the glass being only the part closest to you. There's an interesting teardown of the retina display here on iFixit, and if you want to geek out, you are welcome to read it. My point is that things can happen other than cracked glass. If your computer were on my bench, I would:
# With the computer off, run a credit card or similar thin plastic all over the screen, edge first, to see if I could locate a hairline crack that was otherwise invisible to the eye. If I did, I would feel a little "click" as I ran across it. I would not press hard, just enough to catch any edge. Then, if that did NOT produce any results, I would...
# Boot up the computer, and carefully flip it upside down, holding it by the bottom case (keyboard part), with the screen towards me. Then I would shake the computer a bit, to see if the curved thing moves. Reason: If it's a loose wire that has fallen down, doing this will probably make it move around. If that didn't do anything, I would...
# Shut it off again, find a really powerful flashlight or lamp, and – holding the open computer in both hands so I don't drop it – turn the screen this way and that to see if a powerful light will reveal any anomaly in the screen. This sometimes works to reveal issues with one or more of the underlying layers. You might need to get really close with the light, and run it around using different angles. Pro Tip: You can shine a light in through the Apple logo, as well.
Because that line is semi-circular, I will take a wild guess that someone sat on your Mac and – realizing their mistake – jumped up before it showed any obvious dent in the case. This would flex one or more of the inner layers and create a butt-shaped "wrinkle" in the display. However, I would also not be surprised if a small wire had come loose and was hanging down instead. If I were to place a bet, though, it would be on the Butt Damage option. It would mess up the display without necessarily causing a crack in the glass. And it would disappear when the computer is off (but see option #3, above).
In any case, if this is showing in all apps and Finder, you are looking at a physical issue which will likely involve replacing the display. Taking those displays apart is a royal pain in the part of the body I think sat on your computer, and such "repairs" often end badly.