How many times must the walkthrough magnetometer activate for a passenger to undergo additional screening?

A passenger must undergo additional screening after the walkthrough magnetometer activates twice. This protocol is designed to ensure that if the magnetometer detects a potential threat or anomaly, sufficient verification takes place to confirm the passenger's safety. The requirement for activation to occur twice helps to reduce the possibility of false positives that may arise from items such as metal jewelry, belt buckles, or other innocuous objects that can trigger the alarm. If the alarm activates just once, secondary screening may not be deemed necessary, as the initial activation could be due to benign reasons. Hence, the threshold for additional screening is set at two activations to provide an appropriate balance between security and convenience for passengers.