Something like a proximity mine from a video game. I always did wonder why they didn't just sprinkle a bunch of these in an intermediary zone. Surely it provides incredibly effective area denial. Perhaps it does and you can't get in, perhaps it invites escalation. Definitely seems like widespread automated hunter-seekers will be some kind of terror operation in the next quarter century. Too easy to build, too easy to deploy, and you don't even have to be around when it activates. Controlling the explosives pipeline is surely the only way. I'm surprised it hasn't already happened.