[waste_monk]

I believe they also have cameras watching the scales / scanner platform. If you put e.g. an orange on it and then choose to manually input what type of item it is, orange is usually the first item on the list of suggested possibilities.

Do the same experiment with the orange in an opaque bag and it will still usually have it on the list (guessing by weight) but not always at the front of the list.

I would assume this will also trigger an alarm if you were to try tricking the system by obscuring the barcode on some steak and checking it out as apples by weight (but have not tested this hypothesis for obvious reasons).