Hope you don't mind 610, I've reposted this post from another topic as a new topic as I've got to the bottom of this and thought others might like to know..
Stripping my safrane down today and removing the blower housing, I found this same piece of plastic. It's from the flap itself - stuck to some foam that is supposed to protect the flap.
Here are some pictures:
The offending piece of plastic:
The flap up (closed):
The flap down (open):
Where the plastic should be:
IF you're brave enough to remove this huge lump of plastic (two bolts under the scuttle and then some screws behind the lower dash cover) then you can probably glue this back in place yourself.