Coolant level

[Gnasher]

For a valve type cap I guess it depends which seal fails. If the upper seal fails then coolant will leak out of the cap. If the lower seal fails then it will leak into the expansion tank.

Nope, if the lower seal fails the system can't pressurise and the engine will overheat, it very short (a few mins) order, it won't leak it will boil! If you ignore the temp gauge and the steam/coolant peeing out of the expansion tank all of which will be blatantly obvious.  You'll blow the motor :rolleyes

None of that happend to this chap. 

If the upper seal fails again no pressure same effect as already mentioned and obvious coolant staining around the neck again this chap didn't have that.  Mate on this occasion based on symptoms this chap has provided, it's got nothing to do with the cap.