[rdaprile]

Sometimes when I unplug my S9 from the charger a notification window pops up on my lock screen for about 1 second then disappears. Don't have enough time to read it and can't get it to replicate, no messages, etc. Frustrating.

[SpookDroid]

If you're unplugging while covering the ambient light sensor, it's probably just a notification that touch input is being blocked 'in dark places' to avoid erroneous actions while in a pocket.

You can check to see if you have any notifications (chances are you don't) by turning the screen on before you unplug. You can also use the Notification history activity if you have a launcher that supports those (like Nova). Note that only 'real' notifications show up in that list, not system ones (likely the one you're briefly seeing).