There are some battery draining tricks you can do with Bluetooth apps on a phone to drain the device batteries. Essentially if you did a ton of device queries to every BLE device visible, you could drain the batteries of every device in range. Transmission is probably a heavier battery drain so just making them talk to your phone creates drain. It’s a single device example, but in a bar once I was showing a co-worker…
Do this enough places and over time, and you get a map of what devices are where and at what time, with the assumption that the devices are probably linked to people.
Expand that to a government placing little innocuous BLE sniffers all over the place collecting data has some pretty evil big brother vibes. Or on everyone's phone so we are all sniffers.
But it kind of goes both ways. You could sniff outside a police station and just skim all the BLE devices you can find, and assume with enough data over time the frequent ones are mostly owned by police officers. Then have an app that alerts you any time one of those LEO BLE devices is in range of you. Basically an undercover LEO's Fitbit could give them up.