Yeah I saw that thread, but I feel like it hasn't answer any questions.
Firstly, the icon doesn't go away when I reboot. I even tried switching off the phone for a few minutes and switching back on.
Secondly the usual 4G/3G data icons are vertically oriented, if I disable my wifi and re-enable mobile data, the 4G icon reappears and looks as normal. These arrows are horizontal.
Thirdly, not only are they horizontal, they're also stacked. The incoming/outgoing arrows are individual, coloured green and orange-y red (instead of green and red-red) and appear in the phone logs along with other icons like missed calls and the like.
Edit: I do admit it must have something to do with data though. Because if I enable airplane mode, the arrows disappear. But they come back when I disable airplane mode. Strangely enough, they remain even when I manually disable wifi AND mobile data.