Its reading a vacuum as the car wont run any boost under idle. By the time the air gets to the inlet manifold it will be less than atmospheric pressure due to the inlet tract and therefore read a minus.
N/A cars all run at vacum.
When the gauge reads "0" it meas the engine is getting air at atmos pressure i.e. at same pressure as outside. Its not really boosting but its cancelled out the detrimental effect of the inlet tract.