You may be slowly purging an air pocket created in the system when you replaced the pump.
If you're adding pint upon pint - maybe there is something more. But if it's a lesser amount, then it's probably compensating for the air that eventually will purge in full.
And like you said - inspecting for leaks is the only thing you can really do; there's not a hidden place fluid can go. You should look at the rack and pinion steering gear boots - make sure they are not swollen with fluid, and check the physical connections at the lines. Good luck!