This morning I checked the coolant level and it was about a 1/4 in lower than full. This afternoon I checked it again for no good reason and it was full.The truck was cold both times I checked it and I'm not noticing any oil in the system either. How did a little coolant magically make it's way into my cooling system?

 

Air bubble?
Who knows, I wouldn't worry about it.

 

Air bubble would decrease in size by cooling, expand by warming, even if engine not cranked in between.