Update on this, I believe it was a window asking for permission to either block or allow a program access to something, as I got one today that was exactly like the one I missed, but it was while I was running sophos, and it was trying to scan some protected files, and like the first one it didn't appear anywhere in my notification history.
But the first one also just happened at random, I was not running any specific programs
Now I'm concerned something gained access to my files without my knowledge, or omething was allowed through the firewall, how can I check what programs have been greenlit for what?