Helping people with computers... one answer at a time.
Don't let your computer randomly make the choice of which connection to use. I'll show you how to manually determine which connection is active on your laptop.
Leo, my neighbor sometimes lets me use his WiFi. My laptop is supposed to automatically connect to his wireless network, but I sometimes need some privacy and connect to the internet through my own private USB modem. Can he still monitor my internet traffic even though I'm using my own USB, which is supposed to be authenticated? And can he monitor my traffic if my laptop is connected to both at the same time?
In this excerpt from Answercast #12, I look at configuring multiple internet connections and recommend the proper setup to make sure you are safe on the internet.
I'll put it this way: don't be connected to both at the same time.
If you're connected to both:
a) You don't know which connection your computer is actually using.
b) You could easily be using his wireless when you think you're using your USB.
The safest thing to do is to turn off your wireless connection.
If you have a switch, use that. If you don't have a switch: right-click on the network connection icon in Control Panel and you should be able to click on "Disable" to actually turn the adapter off. That way, you can use your USB connection safely.
No, he cannot see what's going on on your USB.
He can't see anything if the data is going through that USB connection (which I assume is some kind of cellular modem). Then, no, he's not going to be able to see that.
The problem is that (like I said) if you are "simultaneously" connected to his network, you don't know where the data is going. You don't know what he can or can't see. So don't do that.