It was from videogames. The big offender was Rocket League, though Stardew Valley also caused issues.
With Rocket League, it was from the following elements:
The words "BALL CAM" at the lower left, the speedometer in the lower right, and the scoreboard (specifically the orange square where the "1" is in the picture above) at the top middle. The biggest offender was the speedometer -- any time I viewed any content that was yellow, orange, red, purple (in that color spectrum), I could see it. I think Rocket League gave the most problems of any games I played because
A) So much of the damn UI is bright orange, which seems to be the offending burn in color (along with red and yellow) and
B) I played it in HDR at 100 OLED light on average of 1 hour a day.
The other offender was Stardew Valley, though to a much smaller degree:
I could make out the boxes along the lower middle of the screen, along with the top right menu UI. These elements stay static for a good amount of gameplay, and I my partner would play around 2 hour sessions, though only a couple times a week. Oddly enough the "energy" meter at the lower right didn't burn in at all.
I came from a plasma TV (Pioneer Kuro) background and was always very careful with burn-in, so I was pretty surprised I suffered burn in (or, more specifically, burn out) in 1 year on my OLED.
Now that I have a C9, I'm not going to play Rocket League (or anything) in HDR if it has static content of a certain color, which really sucks.
Oddly enough, the game I play the most on my system is NHL 20:
Which has that scoreboard on the bottom stuck on the screen for 90% of the time playing, and it's caused exactly 0 issues. Which again leads me to believe it's the color of the static HUD (along with brightness) that really affects potential issues.