Undetectable Leak
My 1991 XLT will lose coolant similar to yours. I traced the problem to the upper radiator hose. There is a long scientific explanation which basically boils down to temperature change/expansion and contraction. When I first start the vehicle it will leak a steady drop from this hose. It only takes about 1-2 minutes of warm up for the leak to stop. I have yet to replace the hose or clamp (broke college student) however am sure the problem lies here. To get a better grasp on the leak wait till morning, start the engine drive about 100 yards or down to the corner of the street, shut down and look under the vehicle for a leak, on the radiator support for a puddle, around the hoses for a leak, at the corners of the radiator, at the radiator cap, at the overflow, along the radiator for wet spot, at the water pump anywhere coolant flows past. Takes a little effort but this could show the leak, or drop it off the night before at a dealer with a note for the technician to do the same, just driving it into the shop for him will be enough, run it any longer and the leak will stop.
Always check the easiest solutions first then go from there!