Bluetooth security tips (example addresses phones specifically but these tips should be followed for ALL bluetooth devices):
If your phone is sleeping and locked, an attacker can still connect. As soon as they attempt to access your stuff or get tricky and try to take control, the screen would light up - if it's unlocked or you have no password / security, then you've helped them bypass this step. If you have it setup with security (pin, print, etc), then you are safe, at least for now. Don't think for a minute that people aren't working on a way around this because they are - and they will find it.
* Shut Bluetooth off when you're not using it. You're 'probably' safe at home or at work, but if you get into the habit of turning Bluetooth off when you don't need it you won't forget the next time you go to Starbucks. There is no way for an attacker to turn Bluetooth on. At least not yet.
* Make sure you have a secure lock screen. If you don't already have a password, PIN, pattern, fingerprints or anything else set up so your phone is locked until you unlock it yourself, go do it now.
* Turn off trusted devices while you're at it. Tapping in a 4-digit PIN or scanning your eyeballs is way more convenient than getting new credit cards and talking to your bank, even once. Trust me, I've been there. (Thanks, Target. Idiots, I swear.)
* Don't leave your phone unattended. Put it in your pocket or purse and take it with you even if you're only stepping away for a minute or two.
* If you see the screen turn on, look and see why. This is the biggest "flaw" in the hack - it will turn your screen on if someone tries to do anything after they are connected.
There probably isn't an army of people patrolling the streets, ready to hack "all the phones/devices" through Bluetooth. But there could be that one guy, and he could be anywhere (McDonald's, Starbucks, Dunkin Donuts, the library, literally anywhere). In cases like this, it's always better to be safe because the things hackers can do are pretty easy - as are the things we can do to help protect our stuff. Your stuff is worth protecting.
As noted, the latest version of Bluetooth, version 5.0 is rated at 50 mbps limited to 240 meters but the signal gets crappy at about 30'. A Bluetooth hacker attacks the Bluetooth protocol - the device they're looking to hack into may or may not matter much.