If you're sure the undercarriage is fine and all intact, then I would suggest starting the investigation from the boot. Take everything out and have a look to see that the spare tyre and its accessories are in place. Make sure nothing is left unturned.
Then, Once you're satisfied of the boot having been cleared (given the green, they say), move into the cabin.
With military precision, remove the back seat and inspect again for any loose metal particles. Pay special attention to the rear seat buckels. Once you are satisfied, and have declared the area green ... check your front seats underneath. Am not going to ask for their removal as visual inspection will ensure that the bolts and nuts are in place, and no loose metal objects are left unidentified.
That having done, are you sure the doors aren't the culprit?
126 tailed by a 203, 129 leading the pack.