had this problem and in the cases I have seen the fluid level went down fairly quick and made a fair amount of smoke. This smokes so little that it can't be using much fluid. A drop or two entering the booster under high vacuum could easily get sucked into the intake if the booster is like the others I have experienced. Should there be an inline check valve on the TT that is not on the NA? Maybe I am pressurizing the booster under boost and that is causing the seal to then leak a bit under vacuum.
I would expect some to drip to the bottom also or leak out the joint between the MC and the booster. I will loosen the MC tomorrow and check for moisture.