We've been in London for the last couple of weeks, where I've been teaching at a Summer school, and Susanne has been staying at my sisters' (two sisters, both houses, hence correct placement of apostrophe there, pedants).
Coming back, my backpack was x-rayed, as normal. They asked me if it was possible that I had a screwdriver in my bag. I said that it was possible (it was possible that there was almost anything in my bag, but many possible objects were unlikely - I thought that they probably weren't in the mood for debate, so I didn't elaborate on the possibility), and they looked through my bag - finding a screwdriver, which they took away. It was one of my favorite screwdrivers - part of a set from Screwfix.
Then they smeared special bomb detecting stuff on lots of things, and checked that I didn't have any bombs. If they hadn't found the screwdriver, they wouldn't have checked for bombs. So the moral of the story appears to be that if you want to smuggle a bomb onto a plane, don't also put a screwdriver in your bag. (Although S pointed out that if there had been a bomb in a bag that they had actually searched, then they would look really, really stupid.)
A second moral is that it appears (from my very small sample) that your chances of getting a screwdriver shaped weapon onto a plane are much higher at Los Angeles and Calgary (where the screwdriver containing bag had been screened) than at Heathrow (where the screwdriver was detected).