Where does the inductor energy go in a buck converter when the load is suddenly disconnected?
It goes into the output capacitor. The pulse is short enough and the capacitor is large enough that it can be engineered not to significantly make the output voltage rise, maybe by few percent, as long as it is within limits so it's not too high. The output waveform also depends on how fast the regulator reacts to this via feedback loop and compensation as well.