I had the same thing happen: Mystery part magically shows up on plenum cover.
It is a clip for securing the cabling to the driver's side hood hinge. It clips to the part of the hinge that attaches to the hood. I had knocked it loose while wiping down the hinge during a cleaning.
It snaps right back on once you find the target location. Lift your hood, and go look at the hinge; it should be fairly clear how it goes on.