As the coolant in the radiator cools off when the car is stopped, its volume decreases and thus a partial vacuum is created. This vacuum pulls the coolant out of the reservoir tank. If the coolant is not being sucked back into the radiator, then there is a leak that is causing the loss of vacuum. There may be a leak in the top of the radiator. Another possibility, although rare, is the hose between the reservoir tank and the radiator is collapsing when the radiator tries to draw back the coolant. This is rare, but I had it happen on a 1993 Oldsmobile 88 that I once owned. A couple of dollars for a new hose and clamps and 10 minutes of my work took care of the problem.