While I'm running I often let me brain wander in all sorts of directions to pass the time. One thing I came up with was a detective/murder story where the murderer used their fitness tracking application as an alibi, by convincing someone else to run their regular route and discreetly swapping/attaching their tracker to that person. They then do the murder while the other person creates the alibi unknowingly by running their regular route and creating a digital signature of the running.
The case is solved by a brilliant detective combining with a precociously smart computer guy who downloads the raw data from the gps tracker and determines that while route and speed are typical, the data betrays it was someone else because of the difference in gait, recorded by the accelerometer that was built into the device for an upcoming feature not yet released by the tracking company. This is either proved in a courthouse scene where the accused has to run across the room, or by deception by secretly tagging both the accused and their unsuspecting alibi runner and comparing the data.
Or something like that, anyway.
The whole reason you gave your friends access to your fitness tracker data was so they could help motivate you. That's not a betrayal. Additionally, the Pennsylvania suspect voluntarily gave her username and password to the police, without which they would not have had access to intra-day historical data.