"Can it just evaporate?"

No. Have you checked for intermix? If there's no signs of leaks or intermix, your coolant was most likely consumed within the combustion chamber (s) while you were last driving it. pressurize the system & see if it holds pressure. If it doesn't, look for a cracked head or leaking head gasket. In any event (leaks or cooling system leak down), you can look forward to spending a fun filled day getting aquainted with your tools again