Kalali, a shorting plug will have electrical effects, by lowering the input impedance of the unused input to essentially zero ohms. Low impedance circuit points are inherently less susceptible to pickup of electrical noise than high impedance circuit points, everything else being equal. Including pickup of noise that may even originate elsewhere within the same component, that may in turn find its way from the unused input path to the signal path that is being used, or to a circuit point (such as in the power supply) which can affect that signal path.
As was mentioned earlier, and despite some disagreement that was expressed, caps that do not short the input will do nothing other than perhaps reducing oxidation or corrosion of the ground shell that may occur over time, and keeping out a few dust particles.
Regards,
-- Al
