[sundance]

Its probably the water pump. It can leak a little at a time and you not see it. The fan can cause it almost instantly evaporate on a very slow leak.

 

[sundance]

It may have had an air bubble in it and just needed to "burp".

 

[sundance]

Coolant really doesn't evap readily and when it finally does will leave tell tail residue every time . .

You're talking about leaving scale or mineral deposits. Which is true of older systems where common tap water was used. However, most coolant now is premixed and uses demineralized water. If the coolant isn't very old and as such.......hasn't picked up metals or minerals from the system components, then it will not leave hardly any residue behind after evaporation.

Which is why I said what I said. Granted. Most leaks are obvious but I have seen slow leaks that weren't.