I tend to take my belt off for security as a matter of course, because it's got a metal buckle and it might set off a detector with the sensitivity wound up. Given that I'm normally stood in a queue with nothing better to do, it's not a problem given all the other hassles of emptying pockets and sorting out the bag of liquids and taking off my jacket.
However, a few years ago I went through a metal detector in a federal building in San Francisco and set off the detector there. The guard suggested I go back through the gate and try again with my hands over the belt buckle. This did the trick and I passed on the second attempt.