Is it common for the coolant in the overflow tank to be down a little every few hundred miles, or do I have a hidden leak? It seems to down about a cup every other month.

 

The XJ/K series is famous for little leaks on the order that you describe. The best solution is to do, or have done, a pressure test on the coolant system. Most auto parts stores will 'rent' you a pressure test kit for free. Pump it up with the engine cold and you will probably see a little leak.

 

I had same issue and it turned out to be my thermostat housing. No issues after I replace it.