In an open-circuited transmission line (microwave circuits) the current in the return path is completely zero?
A transmission line doesn't have a &quot;supply path&quot; and a &quot;return path&quot;. The two conductors that make up a TL are coupled throughout their length so that they always have opposite currents — this is what makes it a transmission line, and it's true even if you're &quot;only driving one side&quot;, with the other referenced to ground.
The relationship between voltage and current in a transmission line changes along its length; if one end is an open circuit then there is zero current there, but a short distance away the current is nonzero (in a sense, the current at that point sees the remaining TL stub as a capacitor), and a quarter wavelength away the current reaches a maximum.