I am having the EXACT same issue, apparently it has used 56GB on WiFi in the background over the last 3 days as well as destroying my 500MB data plan. On HTC One M8 (T-Mobile) - recently updated to Lollipop.
Hey OP, I'm having literally the exact same problem (the same system apps are eating my data up) - it's the most frustrating thing ever, get through my monthly data allowance in 3 days... Have you found a solution?