Radiator leak

I have a 2003 hyundai santa fe. I recently took it in for an oil change and tire rotation. After I picked it up and drove it 2 miles, or less, I noticed lots of steem and radiator fluid coming from the hood. Mind you, I have not had any problems with the radiator prior to that. I returned to the shop where they said that they added fluid to the resorvoir because it was dry. They found a small crack, on the plastic, at the top near the pressure cap. They say that because the radiator was not full is the reason the fluid didn't leak before. Is this possible or did they somehow put the crack there?