Dirty Bomb on a Plane?

This story about former Russian spy Alexander Litvinenko is fascinating. My sister Nicole has been on top of this for days. Among the latest twists is the fact that two British Airways planes have tested positive for radioactive elements and have been grounded.

But here's an unexplored angle, at least from what I've seen. Why is it that nobody has detected this before the cops showed up? Doesn't this show how easy it would be for a terrorist to smuggle a dirty bomb with radioactive material onto a plane? And how easy it would be to get tons of people to glow in the dark and spread this around?

Most likely no alarms went off (I hope there were some along the way) because of the minuscule amounts of radiation. I hope.

Of course there should be plenty of alarms going off about Russia and Uncle Vladimir, but that's a different story.