[14]

I immediately thought about something like a jailbreak for the device where an attacker could attach a computer, run an exploit, and upload whatever data he predetermined. After the code was run it would leave no trace behind and hard to prove otherwise for the owner.

[akuji1993]

You might also be able to just alter the requests the fitbit sends to the servers to save the data from the device to show what you want (heart rate stopping at another time, differing heart rates, living for two more hours giving you time for an alibi).