I found the answer to my own question.
The circle with 2 dashes in status bar is from the app "TrayVolume lite" (an excellent app).
There is a settings option for the app that allows you to change the 'notification position' to 'hidden'.
I assume the 2 dashes in the circle are intended to give some indicator of what your volume level is at, however for me it remained static with 2 dashes.