if you tether from a mobile phone, it uses a different gateway to the internet versus using a mobile hotspot. so if they look in detail into your account, they can determine if you are using a wifi device or a cellular phone (they each use different gateways). they have been able to do this since vision was born back in the early 2000s. back then counting data was not as big a deal as it is now.
with LTE, they will be able to determine exactly where you go based on the IP's...