HTC One M7 :: Unidentified Symbol On Status Bar

I've recently been getting an icon in my HTC One M8's status bar, which is sort of like a pause symbol in a speech bubble. I initially thought it may have been the sleep mode function which can detect when you aren't using the phone so it turns off the data connection, but I turned that off and the symbol still appears.

It only appears for a few seconds when I unlock the phone and then it disappears, so it took loads of attempts to even get this screenshot. Ignore the other icons in the status bar, I've just got my phone plugged in to my PC. The Icon does seem to apear only when the phone is charging, either plugged into my PC or the mains. The symbol I'm confused about is the very top left symbol.