My understanding is that any movement or rearrangement of the items can trigger the sensors. Thus adding your own purchases even if you don't remove anything from the minibar will record some activity, although I don't know how the system would decide whether to charge you for every item that was moved or do something else, perhaps later charging you for consuming you for stuff you bought but stored in the minibar for a while.

IMO these systems discourage especially higher end purchases from minibars. Who would buy some overpriced half bottle of wine or champagne without looking carefully at the label?

Where the minibar is locked, I decline the key to avoid issues with incorrect charges.