I have 02 wrx just did oil change and spark plug change and added coolant, was driving to work today and my check engine light came on so when i got to work i popped my hood open and seen that my coolant resavoir cap was gone and coolant was everywhere. My cars not over heating or loosening power. Drives fine. What could cause this??

 

Probably did not put it back on right and the pressure build up nudge it off,If it was anything to do with the gasket under the cap you would have coolant all over and the cap would still be on as that is a normal wear & tear problem with the turbo coolant box on the left.