>>33845582 (OP)
I've never seen a bowl left like that. You always had to ring and get the person to give you the candy. What's the point of "trick or treating" if you can't say "trick or treat!" to people?
>>33845582 (OP)
Setup a hidden laptop and digital USB scale underneath the bowl. Write a computer program so that if the weight on the scale drops to zero for more than 3 seconds, then it blasts an alarm.