coolant loss all at once

car is not overheating while driving,, but the last couple a weeks 3 times after driving, severe coolant loss , . cant find where it comes from, and of course when i watch for it , all is fine . Happening as car is cooling down , seems to lose a half gallon or so. any ideas ?
thanks

a pressure test will find most leaks...that are leaking to the outside anyway

check to see if your oil is grey? if so, you are dumping it into the engine block.

oil is fine ,, coolant is going on to my driveway,, and well so is some oil :)

Check your coolant cap. I lost my coolant anf found my cap messed up. I replaced it and so far no more leaks. Good Luck...