I have a 2008 c30 and a couple of months back it broke down because it leaked pretty much all the coolant. My shop fixed the leak that was caused by the upper radiator hose. A few weeks later a similar issue happened. I took it back to the shop and they found that the issue now was coming from the heater house inlet and outlet. It's been good for a few weeks, but today when I was driving it started to feel like it was going to break down again. I was able to get it home and sure enough all of the coolant has leaked again. I don't see anything actively leaking and all hoses that I'm able to see seem fine.

Any ideas?

 

You first step will be to use a radiator pressure tester to see if there are any external leaks.

If the system holds pressure - you may have very easily damaged the head gasket from overheating. Your car does not have a low coolant warning system - so you could have been driving with low/no coolant for a while before it "broke down". How did your car "break down"? Did it stop running? If so that would have been the engine seizing because it was so hot. When your engine overheats - the head will become warped and allow coolant into the combustion chambers or combustion gasses into the cooling system.

A good next step would be to use a combustion leak detector to check for a damaged head gasket. Don't know how many miles on your engine or what your car is worth - but if you have a damaged head gasket - fixing that properly (with Straightening the head, not just surfacing) can easily cost several thousand dollars retail.

If you decide to fix it - there is a company that has a retrofit low coolant warning system that will fit your car.