As it happened on a motorway the first thing I would have done was phone the old bill to attend the incident. Even if they could not be bothered to turn up you would have been given a report log number.
I take it the chap in question stopped, retrieved the fallen object from the motorway then buggered off?
Could you post a link to the dash cam footage as I'm sure I'm not the only one on here would be interested to see what exactly happened.