The above posters are right in that 'excess' coolant will just be pushed out of the reservoir tank when the system expands. But there should always be some left in it, even when the engine is cold (say a couple inches of coolant in the bottom). If the reservoir keeps emptying then you might have a coolant leak out the top of the radiator where the plastic top tank meets the aluminum. That is why your coolant was low in the first place. Do you smell coolant while you are driving?
You should get the engine warm and look under the hood with a flashlight while the engine is idling and the system is at pressure. Look for any places where coolant is leaking. (Be very careful - don't go prodding any hoses or take of the rad cap as the coolant is very hot and could scald you.)
2001 Subaru Forester S, M/T, 2.5L N/A, 165K miles