Coolant leak issues

[D_Train]

My coolant light came on a while ago and I looked through the forum at the BHG symptoms and it wasn't showing any symptoms except the coolant light was on. My car had a brand new engine put in 30,000 miles ago. Today I looked in my coolant overflow tank and saw it was empty. I opened up the actual coolant valve and that looked nice and full, and I have been paying very close attention to my car temperature as I have been driving, so no worries there.

My question is, why was the coolant overflow tank empty? And if it is because of a leak, what are the odds that it is leaking into the engine, or if it is just a leak in the coolant tubing? I called the local toyota dealer and they quoted me $27 to check it out, which would be waived if I got whatever it is that is wrong with it fixed there. But wouldn't it be messed up if it was leaking into the motor after only 30,000 miles and a new engine?