dennis@home wrote:

They can't know the weights of prepacked fresh stuff for a start.

But they can know which items they do know the weight of, and they could
know a sensible weight range for pre-packed veg, I don't use them so
much for that.

Next time you are in try putting a box of stuff on at the same time as
putting say the free magazine on the scales.

Adding or removing an empty carrier bag is often sufficient weight
difference to trigger them.