Open to ideas but I think there is a bug in how data is managed on the 950.
Lol. I am amazed by the weird issues that come out. However, if it is on the 950, it should be on other models also. But I don't see any issue on 6 phones that I have WM10 running.
You can check if the following options are setup correctly on your phone.
1) Check if you are automatically using Mobile Data when you don't have wifi. If yes, click the "Don't Allow" button.
2) Check the phone updates page. It must show the message in the screenshot below. "Updates will not be downloaded over a metered connection".
The next setting is from the store when you have to setup "not to use metered connection" for app updates. With all these settings, the OS should not use data.
Also you can try switching off location for a day or two to check data usage when it is off.