Data depletion

I have heard stories of data going missing. I was even reading about at mybroadband a few weeks ago and I never thought this would happen to me. I go to bed with more than 1 gig of data and at 00:23 I receive an SMS that I have exceeded my limit and I owe more 1000 bucks on a contract that I pay 260 for. When I call the call center they saying someone was connected to my wi-fi and they making that claim without proof. Now I must pay 750 above my normal bill because when I'm sleeping I'm still using my data it seems. Worse I switched off all auto updates from all my devices. <br> <br> I have set a call limit on the number for such things, still they went over that. <br> <br> I have more than 4 contracts with these people. They definitely not going to be with for long that much I promise.