Oh I believe that everything that happened to you happened, I just couldnt believe that anyone would even consider a hand written note taped to their door as an actual official response of some kind.
I probably responded without getting to the end of the thread, and didnt see you say you were not going to do it. I only saw the post where you were asking for "thoughts?" on leaving the car with that person.
Maybe because I am a fairly private person, If this were me, I would be reporting this person to the NHTSA myself, for being so unprofessional as to tape a note to my door in hand written letter talking about an investigation. If it was legitimate, it should be on letterhead. This is not the way a professional organization conducts business.
Either this is an official investigation (and it should be treated as such) or the person is going off on some tangent they are not supposed to (wasting time / money on investigating stuff that doesnt rise to the level of an official investigation), or the person is a liar.
In none of those cases do I see any benefit at all to you for even entertaining them looking at your car, or doing anything other than contacting their supervisor at the NHTSA and informing them of this conduct. Everyone is different, but this is a real invasion of privacy to me, and would be dealt with like I said.