I believe I had an antifreeze leak for at least a couple of months. It wasn't bad and was hard to tell it was present because it pooled on ground in the same place as condensation from the AC. The car is a 2007 S with 62,000 miles. After extended use antifreeze would run down the back of the engine and drip off the transmission. After I removed the thermostat housing the problem appeared to be a leaking temperature sensor located in the top of the thermostat housing. I replaced the whole unit instead of replacing the sensor which costs about $26. The thermostat, built into a plastic housing which resembles a water pump, temperature sensor, and gasket come as a unit from mini for about $140. Pretty easy job, just a lot of wires and other small parts in the way. Three bolts and one retaining pin on a back elbow and it's off. I'm assuming this is a fairly common problem because the local dealer, International Autos in Milwaukee, Wi, had seven of these parts in stock and multiple sensors too. Thermostat part number: 11-53-7-534-521
Just make sure you get all the air out of the cooling system. Sometimes it can be tricky. Keep an eye on the level on the coolant tank as it may drop a little until all the air is removed.
I think they go through a lot of those. I had 2 replaced under warranty - not leaking, just bad thermostats. It's good to see it's not too complicated now that the warranty is up.