Originally Posted by WittSmall
Is that normal? I'll fill it up to the fill line in the reservoir and a couple weeks later, it's at the bottom. I don't think I have a leak anywhere, what do you think?
There are a few things cause that to happen over and over and one thing that can sometimes cause it in a non-recurring fashion.
1) A leak. Could be a small one, coolant hitting hot engine parts evaporates before you get a puddle so you may not see the telltale signs.
ETA: The heater core suggestion below is a good one too. It might be porous and evaporating into the air although you'd definitely smell it in the cabin or worst case have puddle of coolant in the vehicle.
2) Blown head gasket. Usual
symptoms may be white smoke out of the exhaust, coolant in the oil (pull the dipstick, smell, look for a "milky white" goo in the oil), bubbling in the coolant overflow tank while the engine is running at normal operating temp. Any or all of these may be present.
3) Failure to properly bleed the cooling system after a flush/fill or new radiator or thermostat install but that usually resolves itself after filling the overflow bottle one or twice.