plambrechtsen: Probably worth logging a fault with Apple as it is their device that is inadvertently using the data. I know I don't have the problem with my Samsung S4 when I switch off all data.
This. I've heard of this a few times before. Give the 123 team a call, they should be very helpful if you've never suffered bill shock for data in the past. Also log something with Apple or return to place of purchase.
nzbnw