We have come across a couple of cases recently where data usage has unexpectedly increased. This as been a problem as these members connected to the Internet via the mobile phone system and used pre-paid SIM cards. They discovered that over $100 of credit had been used in a few days although they were only using the connection for email. The culprit appears to be Windows 10 update. Microsoft has allowed your computer to share the updates it has downloaded with other computers on the Internet, without asking you. This behaviour is optional and although the default is to enable it you can switch it off. Please see the article in How To Geek for more details.