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That looks similar to the volume indicator in my cinnamon but it's not actually for the volume by the looks of it, the size shape and position's the same but I don't know what you'd call the one on your desktop. I didn't know there were any other notifiers like this for anything else, be interesting to find out what it is . . . . . sorry i can't help though !
I didn't have a keyboard with any settings buttons, but it might have been a button combo-error. The keyboard was at least 6 years old. Replaced it a few hours ago. No pop-ups yet, and I had hundreds before. Marking this as solved.