So here is a picture of Mr. Mystery:
I think THIS mystery is solved! Looking at the sprout towards the rear, it looks to me like these little guys are cilantro seedlings. Gabi must have dumped the extra cilantro seeds into this raised bed after we sowed some in the greens bowl on the patio. So case closed! Or is it . . . ?
As I was photographing this bed, I noticed yet another mystery seedling:
Finally, here's a photo of something that's making me very happy: