Depends on the expansion tank design. I was always noticing a small amount of coolant loss in the Camry. Worried about a HG I investigated for a while. Turns out the expansion tank breather is wide open to air in that car, lending itself to easy evaporation when hot coolant meets the cold air of the tank. Secondly, the expansion tank is very small, which means the perceived % of coolant loss looked worse than it was.
So..If something as small as 2+ cups can influence the level of your fusion's tank that suggests a relatively small expansion tank, and a small % of the total has evaporated over time. Because the tank was not empty, there is no harm, and its better to get the correct coolant to top off rather than straight distilled, especially in cold parts of the country. Its not the end of the world, but my hunch is the coolant hasn't been changed in a while, so its already on the weak side of its life span.
Suggestion: Get the right coolant to match whats in there. Don't play games mixing coolant, not worth it. Your not changing it that much, so the cost and inconvenience of getting the dealer stuff is irrelevant. Take the expansion tank off with the cap/breather combo, take it in the house. Drain and rinse both out in the sink. Once the cap/breather is clean, put it all back and refill. This time go above the full line an inch or so. This will leave less air in the tank which will lead to less coolant loss (hopefully)
