I hadn't actually been in this or any other swamp for a few game days. I was just shooting from the Meadows when I saw an oozer. The only local swamp structure was a Draugr spawner that I had taken out weeks ago. The wiki indicated the fixed Wraith spawns were one time events similar to a troll in a cave.
I'm guessing the Wraith had been out partying with some Greydwarfs and was on its way home when it spotted me.
Reddit had a few scattered reports of daywalker Wraiths but it seems pretty rare.