Yep, you got a leak. Coolant never needs topping off unless you have a leak. This could be external, or it could be internal, with the engine consuming it.
I assume this is a CE or S with a 1.8L?
Look for residue all around under the hood. On and near all the hoses and their connections, on the radiator, etc.. look around carefully with a flashlight, look for wet spots or spots that look like they have been wet recently. A small leak of that size at a radiator tank seam or elsewhere won't necessarily leave a spot on the ground.
Usually small external leaks you will get whiffs of on occasion. Especially when sitting in drive-thrus, as you're creeping along, it's usually hot, and your window is open at least part of the time.