Self-tracking using a wearable device can be fascinating. It can drive you to exercise more, make you reflect on how much (or little) you sleep, and help you detect patterns in your mood over time. But something else is happening when you use a wearable device, something that is less immediately apparent: You are no longer the only source of data about yourself. The data you unconsciously produce by going about your day is being stored up over time by one or several entities. And now it could be used against you in court.