I'm sure that GM will be concerned about the note (tampering), however it is highly doubtful that it got there during the factory processing. The filling and sealing of boxes is very high speed automation with very little human hands-on contact.
The note was most likely slipped into the box after it left the factory. If you take a look at a sealed box of cereal you can actually gently squeeze the box and raise the top up enough to slip in a small note. There is a small dab of glue on the ends, but they could pop open and you may not realize it when you open the box if the rest of the top was still sealed.