My 1994, automatic, 1.0 L, 120,000 mile Geo has consistently had a coolant issue.
The reserve tank remains full (it appears the system is not sucking fluid out when it is needed) and I have to refill the radiator itself at least once every two weeks. When I do it takes a probably a quart to do so.
There are no signs of a leak on the ground or on the bottom side of the engine, radiator, or compartment.
Oh yeah, the water pump
WAS replaced less than 7 months ago due to a leak and hasn't had problems since.