what did I learn about my cooling system today, if anything?

Update: TMR definitely made the call. I've been driving it a bit every day and it's never overheated again. So, I checked the coolant today, and the overflow bottle was mostly empty - not a good sign. But, the radiator was full? Maybe I just timed it right.

Just to confirm - coolant does go from the bottle into the radiator, right? Not just the other way? So, maybe it's still leaking and used up the bottle water and would have started getting low in the radiator if I'd waited a few more days to check?

Or, since there's nothing in it at this point except distilled water - does that boil away some? We're baking around 100degrees here in Houston. I've been holding off refilling it with coolant to make sure the leak has stopped. Wondering if switching over to coolant now would end the loss of liquid.