I was expecting something much more sinister (that this man had a Fitbit and they used it to track his location). This is a much more positive use of the technology, but as a layman, my first concern is that this data can be easily forged.
For example, a murdurer wanting to stick the charges on some other person could steal that persons devices (phone, fitbit, etc). Wear them while commiting the murder. Then return them, without notice if done at night or something.
The other person would then have a very hard time proving that he/she did not in fact commit to the murder, simply because courts have started to rely on these devices too much.
So if a second person puts on your device, and their physical numbers (heartrate, gait, perspiration, etc) differ from yours, you can get a notification, just like when you log into your email from an unknown location.
A good theft-prevention measure, such as locking the device whenever it’s taken off, should prevent such a scenario.
It's more about people putting more trust into smart devices than they should; and the fear this gives me for the future.