OK, I'm going to try that (start the engine, get it hot and look around using a high powered work light). The leak started about two months prior and then stopped so I didn't worry about it. Now, all of a sudden, it is back again. I'm keeping a close eye on the coolant reservoir to make sure it doesn't get low.

Around two years ago I replaced the radiator and all of the cooling hoses. I have a Prestone back flush kit installed on one of the heater hoses; also had one installed on the other car and it sprang a leak (the plastic "T" fitting CRACKED - luck would have it I had just gotten home and noticed the problem - good thing it hadn't happened while traveling); as a result I had to replace several heater hoses on the 560SEL so I'm wondering if this might be the same problem?