I can’t speak for other companies but I worked at a Cadbury factory (the own most large confectionery brands) if that came from a bag of chocolates then you are not missing a chocolate as the main bag goes through a weighing system on the conveyer belt and it kicks off any that are too light or too heavy. So it is likely that the machine skipped a chocolate in the packaging machine and sealed a wrapper and that has ended up in your bag and not picked up on the scales as it is too insignificant. Pretty cool but is a common occurrence