BLE communication can happen inside of a "connection", or outside of a connection.
A typical device "advertises" it's presence with beacons which are broadcast on 3 channels. These beacons are user-defined, so you can use them like UDP packets. Sensor wakes up and broadcasts the current temperature.
Your smartphone can receive advertisements while scanning. Check out the insane number of beacons present in an American apartment complex.
Note: BLE connections may be encrypted, or not. That's up to you. You do not need to "pair" (exchange keys) to communicate with a connection. There are 4 "modes" to authenticate. Without an out-of-band communication mode, all are vulnerable to MITM.
The latest BLE standard improves range with a half-data rate PHY. Range is determined by transmit power and attenuation. Most BLE radios are designed for short-range communication. I've never seen one consuming more than mW, but that does not mean you couldn't make an amplifier that transmits BLE much further