2007 Chrysler Sebring cooling system leak

Hi,I have a leak in my cooling system but I can not see any visable leaks. On the ground udner the engine will be a small amount of antifreeze. I took it to a mechanic he said it was a T-stat and control valve leak. Sense I can not see any leaks when I look at the engine this does not make sense to me. Is the T-stat and control valve not visable? I called a Chrysler parts dept. They think he ment a thermostst housing. But isn't that visable? Plus sense I brought the car back from the mechanic it shows no signs of leaking, but I do slightly smell antifreeze when I look under the hood. Can their be a leak under the engine that I can't see. Under the car is a schroud that covers everthing so I can't see anythig Thanks Ken