In response to “British parliamentary traditions,” Pottsy at Fear of the reaper commented, “I used to work at the U.K. Parliament and sometimes gave tours of the building and would explain the State Opening and all the traditions that go with it, including checking the non-existent cellars (by banging on the floor) for gunpowder.”
Non-existent cellars? I asked. “The cellars they check don’t actually exist and haven’t for at least 150 years,” she wrote back.
So there you have it: They’re not only checking for an impractical and antiquated explosive, they’re checking for it in a place that no longer exists.