That amount of leakage should be enough to leave some kind of trace - or puddle. Put a sheet of cardboard under the motor to get a fix on location. The water pump upper "O" ring seal can leak slowly enough for the coolant to evaporate before it gets too far. Heater hoses and WP to heater pipe (under the exhaust manifold--it can rust through) are difficult to see and require good light and mirrors. Check your transmission fluid (if automatic) for coolant contamination. Examine the carpet on either side of the tranny tunnel - remove the side trim for a better look. Pressurize the cooling system. -- Dave