Get a Clue - I've finally figured out something about these airplane terror alerts. I always wondered what good it did for authorities to let a plane land, and then question the passengers. You would think that once a plane arrived safely, there'd be no more threat. That's true, if you were only looking for a hijacking. But according to this WaPo article, authorities are actually more worried in these recent scares about the use of chemical, biological or radiological weapons. The hint came in this paragraph:

Some of the nation's highest-risk and highest-profile airports have the capability to detect radiological materials, usually through an agreement with local law enforcement personnel who are trained to use such devices, said Carter Morris, vice president of transportation security policy for the American Association of Airport Executives. Morris said radiation detectors have been used on aircraft while passengers were kept on board.

So that's what the security forces have been doing when they sequester a plane on a distant portion of the tarmac for hours before allowing unloading. Of course, I'm not sure what good that does against an undetectable biological agent - although I guess authorities obtain full background checks on the passengers and possibly tracking information to keep tabs on them in coming days. Hard to say what more they could do.