I suppose the crack could be in the block somewhere as it sounds like they swapped the head gasket out correctly. Then again, to really check the head you have to put a really straight edge on it to make sure its not warped. But I bet they did that.
As for the cap on the radiator, that would be more if coolant was found in the water - i.e. it built up enough pressure to back up water into the cooling system. You could look for foam on the dip stick and milky looking oil. That would be a definite problem.
On your head, I take it there is no chance of valve cover gasket leaking down and collecting where you might see it?
