2006 Nissan Sentra pinhole in the condensor

My air conditioning started blowing hot a few days ago. I have the extended warranty on my car. I took it to a car dealership to have it looked at.
The mechanic called me a while ago and said that I had hit a rock and put a pinhole in the condenser and road hazard isn't covered under the warranty. It would cost me almost $800 to fix.
I asked him if it was something that had happened a while back and maybe it was a slow leak. He said no, that he could guarantee that once the hole was there, everything leaked out at that point.
My question is, how hard would a rock have had to hit it to put a hole in the condenser? Wouldn't I have noticed it?