There are several preconditions. It's not a computer message; think 1970s tech. There's a accelerometer to say you need a head-on crash and a seat mass sensor to say there's a large enough human in the seat. There are cases where the passenger bag did not deploy, for example a stuck-closed accelerometer, and then a responder knelt on that seat getting to the driver, satisfying the last condition, and causing a deployment with obvious injury.