In-direct plus is a sensing system that, in addition to its indirect sensing functionality, has the ability to detect the presence of a live person inside the vehicle by means of sensing human attributes such as movement, temperature, sound, weight or any other sign of life. Direct sensing system detects the absolute presence of a human inside the vehicle by means of tracking heartbeat, respiration, movement, or any other sign of life. Indirect sensing system derives the potential presence of a subject or object inside the car based on logic using information such as door opening, pressure or capacitive sensing.

The IWG is discussing the time interval from door lock to initial warning, with options of 15, 25, or 30 seconds as specified in para. 6.4.1. Investigation of door vibration effects during closing at three speeds (strong 3 m/s, medium 2 m/s, weak 1.2 m/s) on a medium SUV showed door vibration diminishes around 10 seconds. The system requires at least 3 times respiration cycles for accurate child detection judgment to reduce false-positive warnings in the field. A 25 seconds delay is recommended to allow appropriate time for accurate detection while accounting for vehicle vibration disturbance.