Fast track pro: signal on output 2 while output level at 0

[sbrunot]

I'm using Fast track pro, driver 1.9.4, on mac os X 10.9. I have a set of krk monitors connected to unbalanced output 1&2. When I set the output level knob to position 0 (no output), with the mix knob full right (no input monitoring), I'm still hearing sound on output 2... When I turn the output level knob to the right, I begin to hear sound on both outputs.

Is that expected ?

Thanks for your help.