Sawman - airliners always have enough fuel for a divert, and if they had an issue they could still call "mayday" and would be allowed to land, even in the middle of a security alert.
On one of those fly-on-the-wall programs, they evacuated the airport after a passenger had an ashtray that looked on the x-ray machine silhouette a bit like a grenade. The passenger was more than happy to open up his bag and show the security guys that it was an ashtray, but they wouldn't let him. The airport was closed for hours. Sometimes some common sense would help...
However we don't know the circumstances around this. To be honest, I'm surprised it doesn't happen more often!