... But it seems to me one could establish fairly quickly that this girl was no threat, just young and ignorant.
I beg to differ. I find your remark extremely naiive. She may be young but she is old enough to take responsibility for her actions. I certainly considered myself an adult at 15 years old and would not have dreamed of being impertinent to "officials".
When I served in Ulster, I saw a 24 year old mother in Northern Ireland with a bomb hidden under her baby - so forgive me if I find your statement rather naiive!
Unfortunately for the world, the terrorist does not necessarily wear pyjamas and a beard. He/she could equally be a well-educated, well-bred English aristocrat. We all know how easily College and University students are influenced by "causes" and some could be possibly persuaded to smuggle a device on to an aircraft if they believed the "cause" was right!
Every potential threat has to be taken seriously. "I have a bomb in my bag!" could be the girl's way of alerting security that she was doing something under duress. Can you imagine how the gutter press would react if a reporter said "I have a bomb in my bag" and the Security Guard took no notice!!!