Overheating

So I just bought this yesterday. It started up just fine and it is quiet. I drove the car home and I noticed when I opened the hood of the car that there was antifreeze everywhere. Funny thing is it was coming from the over-flow part. Now at this point I am thinking that it is because of the top not being on there tight but to my surprise it seemed like that was the wrong top. Any who, I take the car to my mechanic he said that there was no visible leaks and that maybe it is the radiator cap because it was burned. So he told to go get another cap and see if that works. Sadly it did not because I was on my way to work and it did it again. But its not showing on the dashboard that it is running hot.