Did a port scan the other day and was surprised to find my phone was listening for connections on the LAN. Port 9000 on tcp6, which Nmap decided to call CSListener.

Nmap scan report for Host is up (0.0072s latency).
Not shown: 999 closed ports
9000/tcp open cslistener
MAC Address: C0:EE:BE:EF:BQ:15 (OnePlus Tech (Shenzhen))

Googling didn't turn up much so I ran an adb shell to the phone.
Turns out that the process occupying the port was the Pebble Time app.

root@A0001:/ # fuser 9000/tcp6 1417
root@A0001:/ # ps | grep 1417
u0_a123 1417 253 1779444 105092 ffffffff b6db5340 S com.getpebble.android.basalt:framework
root@A0001:/ #

Then I remembered seeing an option in there ages ago about allowing developer access, which I did when I was playing with creating tutorial apps for the watch. The 'Developer Connection' was still switched on, turning it off kills the listener on port 9000.

Pebble Time App