I'm using a tethered internet connection right now. Can T-Mobile find out?

Yesterday I transferred my Iphone 4 from AT&T to T-Mobile. I'm a prepaid customer and i'm on the $70 Dollar per month plan which states no tethering.

However, I can activate the Personal Hotspot to my amazement. I had my phone activated in the store and they left all of my user APN settings empty and my phone still works much to my surprise.

However, due to the slow settings T-Mobile gave me alternate settings to plug into the phone and they gave me tethering settings too.

Here is my question.

Should I leave the APN settings blank as they don't do much anyway and continue to tether and will this cause T-Mobile to miss my tethering in their system or should I continue to use tethering with the settings plugged in. (My phone is an unlocked device)

And if T-Mobile finds out how will they bill me as i'm prepaid. They don't even have my correct name, address or any personal information on me.

Please advise.

I'm a little curious as to why they don't have your correct information

Yes they will find out.
they will shut of your service.