I got another recent problem I just discovered which I was not sure about before. I had a sim card from Smartone in Hong Kong of which I used in my iPad mini for Data connection. Before departing Hong Kong to China I switched of cellular service on my iPad. A few couple of days later I used the iPad via wifi for days 2 until 4. I get back to Hong Kong and found a huge bill for roaming 😱 of which gave me a negative balance on my sim card. I was almost positive it was off the whole time as well as I had personally switched it off. This time I left hong Kong and headed over to Thailand and was going to make sure that this did not happen again and switched everything off again. I get to the Hotel and found that I had a local Data connection on DTAC and that I was roaming with roaming data charges used. I was really pissed and took out the SIM card out of the tray.
Conclusion: Either Smartone SIM cards are sneaky and turn themselves on while abroad or there is a bug in the OS which turns the SIM card Cellular service on by its self.