Have a repair shop pressure test your cooling system to check for leaks. If you're loosing coolant and there are no visible leaks, its possible the coolant is leaking internally in to the block itself or a cylinder.
Thank you for the follow -up! I did have a pressure check on the reservoir itself, by bulb screw on and pump-up. No leaks found. I don't have any obvious fluid in the oil pan, as seen on my dip stick. Is there another check to evaluate the cylinders?