data disappears even with wifi router off

I am hoping someone can assist. We have been suffering from 'disappearing data' since the beginning of the year, ever since taking out a new vodacom data contract - sim number ********** 583. I have gone through all the devices, stopped automatic updates, installed network monitoring tools, changed wifi router password, removed devices one by one, all to no avail. We now only have one phone on the router, and I only browse a news website for about half an hour each evening. Yet still manage to burn through 5GB of data in a week. We keep the router off when not using it. I have logged a call previously with Vodacom and got itemized billing, which just proved there is a hell of a lot of data being used up.

Then this past long weekend I experimented -

Fri 21/09/2018 Router off Bought 3GB prepaid data for R299 at around 17h00 Using mobile data checked vodacom balance - was at 3.00GB Switched router on and went on News24 website using android phone for about 10mins at around 19h00 - switched router off afterwards Using mobile data checked vodacom balance at around 21h00 - was at 2.44GB Router stayed off throughout the night

Sat 22/09/2018 Router still off Using mobile data checked vodacom balance at around 07h00 - was at 2.44GB Using mobile data checked vodacom balance at around 13h00 - was at 1.57GB Router had not been switched on at all that day! Phoned vodacom 082 135 and logged a service request - was promised a response in 72 hours. Technician says 88% of our usage is going on youtube, someone watched youtube from 07h30 til 09h00 that morning. We of course do not watch youtube.

Sun 23/09/2018 Router still off - not switched on since Fri evening Using mobile data checked vodacom balance at around 14h00 - was at 0.83GB

Mon 24/09/2018 Router still off - not switched on since Fri evening Using mobile data checked vodacom balance at around 08h00 - was at 0.34GB

Is it possible that my SIM is cloned or there is someone else out there with the same SIM number as me? Or is it possible that someone (ad Vodacom?) is skimming our data? What else could explain it.

Thanks Alison