When I look at tracking reports and see Wifi traffic, then expand that it shows the ISP's. So for example Wifi > T-Mobile.
Does that mean the user is using T-Mobile but it is not their own ISP/Carrier? But rather it is the ISP of the place that is offering the Wifi?
No.. you either use 3g or Wifi..
Of course not. Your WiFi will go through your WiFi chip to AP and to some other ISP. There is no connection with 3G just like you can use WiFi even without SIM.
But the user's carrier can(though highly unlikely) use some tracking program to track him and find what he is are doing. Even if they do, the WiFi usage won't be counted as 3G.