I noticed on my 2000 Avalon XL that the coolant level in the overflow reservoir drops a couple inches every year. I understand that temperature affects the level reading; however, I usually check first thing in the morning and compare the readings in similar ambient conditions (i.e., similar weather conditions, etc.). I've had the system flushed once already and pressure checked by the dealer, and no leak was found. Is a slight coolant drop a normal part of all cars or should the amount of coolant in the system remain constant between flushes when there is no leakage? Personally, I don't understand how a closed system like the coolant system can lose fluids when there is no obvious leakage. I would appreciate an explanation.