Looks like there are two different options to put Bluetooth in discovery mode:
First method:
$ sudo bluetoothctl
#discoverable on
#pairable on
#agent on
#default-agent
Second method:
$ sudo hciconfig hci0 piscan
Second method seems to be simpler. Is there any problem in using the second method? When does the first method make sense?
The first method should be used going forward because hciconfig has been deprecated along with a few other command line tools. I believe they are still bundled in your OS either because you have an older version of the OS or maybe because there's a grace period before the tools are fully removed.
You can also use btmgmt to get your device to be discoverable/connectable/pairable. The difference is that btmgmt is considered to be more system related while bluetoothctl is application-related. See this answer below for more information:-