Pay him attention, he is right, and I am also a mechanic and looks at the water pump.
Give thanks that you have not loaded the cylinder head for the warmth. The next time something like that happens to you, with which it warms the first time, because the water of the expansion vessel does not go anywhere unless you have a serious problem.
So if you see the empty glass from good to first = fat problem. stop and let cool and right to workshop than if not even the fattest.
Anyway, it has had to throw water little by little for some time. Because these things "warn" and I already tell you that the water does not disappear because yes, in addition, on the right side of the engine, below and near the transmission of the right wheel, you will see pink or green red traces of dry antifreeze because it has thrown it x there that is where the pump is.
Greetings and invites to dinner at your colleague ....