When I had burn in, sometimes the affected area looked darker, other times it looked lighter, mostly dependent on the color being displayed on the screen. Mine was from the notification bar when used in landscape mode, Mine was an 1/8" stripe down the right side of the display. Since then I've trained myself to switch things up in terms of the orientation I use the phone in, to try and prevent it from occuring again.
When this thread first started, I assumed it was the area between the notification bar and the screen when using a black wallpaper. There's a lighter shade of black there, which I'm assuming is so the user can see the edge of the notification bar. This was present on my Pixel 2 as well, and I though it was a defect, but the tech I spoke to said it was there on every Pixel 2 they'd seen. So it was there by design. Perhaps this is either intentional or a byproduct of the notification area, I don't know.