Is there only one SMBus on PC Motherboards?
Have a look at the datasheet for the ICH10 family.

On page 214, it describes the SMBus. There is only one SMBus interface onboard for controlling/polling motherboard devices, but there's an additional slave SMBus, through which an external master device may send events to the ICH10. There's no way to use this programmatically from the computer's side. Section 5.20.7 on page 222 describes how this works and what you can do. It looks like it's used for lights-out management, external watchdogs, sensor telemetry, etc.

It could well be you've attached to that slave interface, which is why you're not seeing traffic. The pull-ups on those pins are probably there to protect high-impedance inputs.