Modeling input bias current in op amp
Here's a simple opamp input stage with 2 bipolar transistors (I didn't bother drawing the rest of the opamp). The bases of these transistors are the two inputs of the opamp.

So, bias and offset input current, which is the base current for these transistors, always travels in a loop, like all currents do. Since it's a loop, it has no &quot;start&quot; and &quot;end&quot; points, but it helps to always start with ground. So, the loop is
Ground
-&gt; through caps and power supply into your power supply +/-15V rails
-&gt; into the opamp via the power supply pins
-&gt; through the internal circuitry
-&gt; out of the input transistors bases
-&gt; through Rc
-&gt; and back to ground.
The ideal opamp model doesn't have power supply pins, so it does NOT respect Kirchhoff's circuit laws : the output current (and input bias/offset currents) just magically appear out of nowhere. I think that explains your puzzlement. But in a real opamp, of course the input and output currents loop through the power supplies.
Note that if you use SPICE to simulate a circuit, many opamp models will cheat and reference ground internally, which means the simulated current it draws from the power supplies will be incorrect, among other problems.

there should be a (theoretically) measurable current at the input terminal.

Yes, you can measure it. It's tiny, so you need a high value resistor. A simple way is to set the opamp to a high gain, use two different values for Rc and measure the change in output offset voltage. So you use the opamp to amplify its own input bias current.