I used to use a kubotan as a keyring, which I carried pretty much everywhere, including past the TSA checkpoints at airports. It looked like this, except that mine was pink:
It didn’t get past the TSA every time. On my way back from the Fitness for the Fighting Arts seminar in Virginia this year, they searched my bag after it went through the x-ray and dug it out. Then they confiscated it, checked my record, filled out some paperwork, and told me stories about their mate who was really good with the kubotan and could “just put yer on the floor, just like that.” (Oddly enough it was one of the more pleasant interactions I’ve had with the TSA. It was certainly better than watching them yell at some old lady for not getting her shoes off fast enough.) And then just before the flight took off, one of the stewards came up and said “they just wanted me to check – was this your assigned seat?”
Reading the Darren Day story, it looks like I got off lightly. One of the people quoted says about the kubotan: “the worst-case scenario it could kill somebody.” I think that’s probably right, but then, so could the floor, or a t-shirt, or a walking stick. Safety is hard. Both for me, and for the TSA.