I was at a bmw panel beater the other day and was looking at the airbags detonated in various different accidents.

Firstly it appears that the passenger side head airbag often detonates without there being any passengers on that side of the car (im assuming this because the front passenger side door airbag hasn't been detonated). Why does it do this and under wat criteria does it detonate?

Secondly on cars with rear side airbags these often seem to have detonated as well but not that many people have passengers in the rear very often, so do these operate with the front side airbags regardless of whether or not there is a passenger in the seat or is there a sensor in the seat like for the front passenger airbag?

Also am i correct in assuming that the sensor in the passenger seat that control whether or not the front passenger airbag should be detonated also prevents the side door airbag from being detonated unnessesarily?