A POS-based checkout/scale system having (i) a bar code symbol reading subsystem for reading bar code symbols on products being purchased at a retail POS station, and (ii) a produce weigh scale having a weigh scale assembly for weighing one or more produce items on a weigh platter during produce weighing operations carried out at the time of checkout at said retail POS station. The system also includes an automatic produce weighing interference detection subsystem, supporting an IR-based light curtain about the weigh platter, automatically detects when any object is overhanging the weigh platter during produce weighing operations, and generates an alert signal when such conditions are automatically detected.