Cellular data - usage keeps rising!

I'm using a new iPad, with a T-mobile sim.

I've got an issue I have seen elsewhere on the forum, that I can only get 3G data to work after a restart.
So that's why I'm finding this next issue so strange - I appear to be using so much 3G data.

I reset statistics a week ago, and the device is now claiming I have used close to 2gigs of cellular data, even though I haven't used the iPad outside the house (where it's on WiFi.)

I've switched off cellular data now to make sure that doesn't continue, but any thoughts as to how this might have happened?

 

Spotty wifi seems the most likely culprit, but for such a high amount of data, it must be video streaming, and I would have thought I would notice the video quality difference.
Also - as I normally need to switch off the iPad before the cellular data will work, it's even stranger.