This is a very data intensive phone. Not really sure what tell you, but I'm not surprised it's "sneaking" data on you...that's kind of what it does.
Do you have the GPS turned off?
Opening Google Maps is going to use some data, it has to do something to ascertain your position, whether that be your GPS, or your 3G, or usually the combination of the two...it's not just going to go off of the WiFi.
If you want to try another Sense 3.0 ROM you may want to try Community ROM...its very smooth and run's very similar to Nils' Business Gingersense.
[ROM][8-18-2011] Community ROM RC5 [Sense3.0][720p][CRT][Battery%][Overscroll glow] - xda-developers
As I said, its a good Sense 3.0 ROM..not perfect mind you, but then, neither is the official Gingerbread release (what are we up to, delay number 3 now?)