> I'd think passive data collection would be more interesting (for nefarious purposes), considering that BTLE is widely used for sensors and in particular for sensors that record very personal data like heart rate.

What nefarious things can you do with knowledge of someone's heart rate though?

Spoofing HR and power telemetry for opposing running/cycling/triathlon teams. I'd send it slightly low, say 3-5bpm, so the target overexerts themselves.

While it doesn't necessarily use the heart rate data (maybe some ID gets broadcasted), you probably can use it to track somebody's whereabouts... to know when they are close to a specific location, for example.

Paparazzi spying an ailing Senator, for example. BTLE is also used for more sensitive medical sensors.

If you detect irregularities you can deny them health insurance.