I've searched this forum and nothing concerning the Engine Coolant fluids...

Yeah so I checked the Engine Coolant Resorvoir today and the fluid level was below "MIN." After whiggin out a little bit, I went to my nearest autostore and bought some "PEAK 50/50 Distilled Anti-Freeze Coolant" (It's like orange colored) and poured it in the Resorvoir. I let the engine run for a few minutes and turned on the heater to MAX. The heater instantly defogged my windows, almost like a explosive rejuvination. Is this normal? Why did the coolant level go below "MIN" in the first place? I know some of it is still in the engine....but aren't the coolant fluids suppose to go back into the Reservoir? Maybe like a leak?