You may experience some instability when using the scan feature for extended periods of time in high-traffic areas due to an error where the bluetooth adapter enters a bad state. The only way to recover from this state is to power cycle the adapter or reboot the machine.
To increase scanning stability you can lower the USB speed on the RaspberryPi to 1.1 (the default speed is 2.0), but this may cause USB 2.0 devices such as some keyboards and mice to stop working.
To lower the USB speed on the Pi, edit the /boot/cmdline.txt file and add the following entry: dwc_otg.speed=1 After you reboot the Pi these changes will take effect. After lowering the USB speed, you can also attempt to recover the Bluetooth adapter from this bad state (should it happen again) with a reset:sudo hciconfig hci0 reset