Thanks very much to both of you.
We don't really use the front trunk, and my wife did open the rear trunk last night, so that definitely makes sense. I had thought it was probably an internal part, based on the fact that it is quite clean, except for the rusty part. When she gets home I will check it out, and see if I can replace it, or what parts I may need to order.
Again, many thanks!
Edit: And I have no idea how that would have fallen out without my wife noticing it / hearing it hit the ground, etc. I'll have to ask her about that!