OzPax1: Yes I've been through LHR and had my checked baggage x'd, and matched. But did the x-ray include an explosives scan?
You had raised a good point about concerned pax mistaking an Air Marshall for a terrorist during an onboard crisis.
Boeing: The Air Marshall couldn't sniff out a suicidal bomber. As I said before, a suicide bomber does not require to announce his presence, nor does he require access to the cockpit to blow himself up or to remotely detonate his checked suitcase bomb while aboard a fully fueled and loaded 747 that's climbing out over a large metropolitan area.
Suicide bombers don't confront people, soldiers or police, they just get on a bus or into a crowded mall and blow themselves up and everyone in proximity. That's why it's important to include explosive scanners at all pax and checked baggage screening points.