Sigmund, you must understand that all the cellular companies are wholely owned Al-Qaeda subsidiaries. They are terrorist organizations.
Huh? No, seriously, the ceulluar companies rely on terrorizing their rubes, ergh, customers, about everything. They ALL claim that they can and will detect tethering and if they detect tethering--or streaming audiovideo, or wifi sharing, or anything else that they offer to sell you at extra cost--that they WILL CHARGE YOU FOR IT and charge you for the highest rate and longest period.
Sure sounds like terrorism to me.
Can they catch you? Probably, since all internet traffic consists of packets, and each packet has a header identifying the source and destination and sequence, so it is fairly easy to run the packets (or the packet log that every server keeps) through a program to sort, process, and identify what they were.
Will they BOTHER to catch you? Given what you hear online, it sounds like they routinely audit random lines, like an IRS audit. Maybe it is one in ten customers, maybe one in a thousand. Lots of folks get away with it but the answer is YES, they CAN CATCH YOU if they bother to try.
Supposedly apps like PDAnet can't be caught--since they run on the phone and reroute the traffic from it. But in practice, someone could still look at the traffic, analyze the traffic, and conclude "Yeah, that's going someplace besides the phone" and charge you for it.
Worry, don't worry, argue, pay, don't pay, your choice.
I'd still say to call Homeland Security and report it as domestic terrorism.<G>