After initial investigations, nothing became apparent. Until she had issues with connecting to the wifi of our home network and her iPad connected to her personal hotspot on her phone without it being switched on!
I will admit it took me by surprise, but it seems Apple introduced a new feature which automatically allows devices that share the same iCloud account information to use personal hotspots on devices, whether they are switched on or off and whether you change the passwords or not.
While there is a great debate about the security issues involved in this update (for example, when your personal hotspot is switched off, your device is still broadcasting it's SSID for all to see) the main issue for us was the unknown costs.
For while my wife was out with the children, in a location that had no wifi, the iPad found the hotspot which was switched off on the phone, activated it and used the data on the phone without her knowing it was going on.
Yes, it advertises a big blue bar across the phone when a device is connected. But if the phone is in a bag within range of the two devices, it can all happen silently and you could find yourself going over your data limit.
You can't switch it off. If you switch off the hotspot, the other devices can switch it back on.
The way we managed to prevent it from searching and finding the iPhone, was by turning off the BLUETOOTH on the iPad which prevented the two devices from covertly talking to the outside world.