I'm from Singapore and I have not seen a store with sensor at the exit. Let's say that if I paid at a counter and I leave the item there after that the cashier would call for me. Similarly if I paid using a self serve checkout as there would be staff around.

Assuming that I'm at an unmanned store it would be monitored by cameras and AI. From those that I have seen on videos after paying I would need to exit the store. If I were to leave without my Redbull like you said and return to grab another I think the system would just take it that I am performing a new transaction and flag out that I'm leaving the store with an unpaid item.