Let’s see if we can figure this out. Anyone want to give the odds that it’s some joker at the packaging plant having us on? 100%? A note got put in the bag before it was sealed.
Really hard to figure out. Send me the reward please. /S
Have you ever watched how food is packaged? Human hands never touch cereal while it’s being packaged. It’s all done by giant machines. I don’t know how anyone could get a note into the part of the machine that positions the bags into the filler mechanism.
So it is possible for someone to open both box and bag and reseal them then slip them back into the stock to end up on the shelves.
The last part is the trick.
Doing it as a customer is not likely. Getting it into the store without being seen would be difficult. BTW this is why if you return food to a store it is thrown away. They do not want to put product back on the shelf that may be poisoned in some way.
Next is a store employee. Possible but not probable. They would have to sneak the products somewhere where they would have privacy to do it. To small an area, to many people around, way to many cameras.
Since we are talking about various packaged foods across various stores I would check the central distribution center.
Probably the easiest place to do that and the least watched.