Coolant disappearing from reservoir

This sounds like your not bleeding the cooling system.when air is traped in the system, pressure builds up forcing the coolant out. You need to find the bleeding nipple,often times located near the top radiator hose.(in the area where the radiator hose is connected to the engine)open the bleeding nipple while poring in the coolant,until there is a steady stream of coolant flowing through the nipple.All this should be done with the engine running.

if this happened only once, is because (like romyspec said) the system needs bleeding and the system needs to be topped off with more coolant.

this car has no bleeder, remove heater hose at y in back of the engine and fill. Run till hot then allow it to cool then top off.