Help please?!

Did you call your service center? How about a picture?

Really difficult to offer help based on this. Maybe someone else is more willing to hazard guesses, but you should just take it in.

Can you provide a closer, in-focus picture? Really hard to identify anything in the current picture.

Ok, on my car that thing hanging down is a plastic screw. I touched it with my finger and it is completely loose - it moves freely. I don't understand what its purpose could possibly be. But my car's annual inspection was done two days ago, so surely if it was a problem they would have flagged it. My car is doing fine right now, no problems that I know of.

Service center will take care of it, whatever it is.

I think the issue with the picture is that the "thing" in question is over-exposed and washed out.

Regardless, from what I can gather in the picture, it looks like you hit something metallic, and a shred of metal off of whatever you hit stuck to the underbody of the car. You can hit surprisingly small or lightweight metallic objects and still hear quite a thunk in the car. If you inspect the underbody and don't see any damage to the car itself, I wouldn't be too worried about it. If you see underbody damage, definitely take it in to have it inspected.

But again, from what I can see it looks like you're seeing a chunk off of whatever it is that you ran over.

- - - Updated - - -

Ah ok the latest posts came in as I was writing my last post.

Based on that, whatever you ran over looks to have just torn some metal from the screw, like a piece of string cheese. Probably not a critical item, but call your service center in the morning and seek their recommendation. My guess is that it's something that can just be replaced at your next scheduled service visit....or maybe the shred can just be removed, no harm done.