"The question then becomes, will the coolant level return from MIN to MAX as the temperatures rise at a later date, in the spring say?"

..........maybe, maybe not. Although you say it lowered that much in one week are you absolutely sure of that? Did you physically inspect the bottles level (note it) then see its level drop that much in exactly a week OR........is it possible that you didn't really observe the level at all until the light came on, assuming it was much more full before?

I know I'm nitpicking here but trust me in that I see and top up coolant bottles every single day of my working life and neither I or the customers ever give it any thought. It's especially common this time of year to be adding coolant to those bottles and these cars will go well over 100K+ without any serious coolant related problems.