I subscribed to this thread when the first post was made because i wanted to see if anybody had the answer.
It has been talked about before on here and i couldnt figure out then how it could be happening.
It was always the rear lights being talked about, but after seeing the headlight photos it seems even more impossible now.
They way i see it.....with everything in place the front and rear lights are effectively sealed, no bugs could possibly get in.
This just leaves one option - the bugs must have got in during the changing of bulbs. It doesnt matter who was doing it or when it was done, but the important thing is that the unwelcome visitor was not noticed at the time.
This seems like the only logical explanation to me.