Weird, didn't know they did that. On literally my first flight after getting pre-check, I somehow had a razor blade stuck under the insole of a pair of shoes I was bringing in my carry-on bag (a pair of shoes I hadn't worn since before I had moved apartments, so I assume somehow that's how it got in there). The metal detector picked it up, the TSA agent took my shoe apart, found it, seemed totally unconcerned, threw it in the trash, and waved me through.