I would presume that an SMPS when off load only draws enough input power to keep the output stage at voltage. Off load the efficiency will therefore be 0. The current drawn from the mains will be enough to compensate for leakage in the output capacitor(s) and this will depend on the design of the unit and the quality of the output stage electrolytics (in the main).

Similarly in a transformer based supply, in an off load situation the self inductance of the primary will limit the input current, but not as much as an SMPS in the same state. Again the idle current will be dependant on the transformer design.

Are these valid assumptions?

Rob.