1991 300 wagon - faint coolant smell

[86560SEL]

Hi all. My uncle bought a 1991 300 wagon about 6 months ago (with the M103) engine, with 147K. The car runs perfectly, but today when driving (with the heat on for the first time) he said there is an occasional and very faint smell of what he thinks is coolant (anti-freeze). NOW, the car does not leak coolant (at least the level in the tank stays the same), it runs in the normal temperature range (around 80-90°C), no oil in coolant or coolant in oil, etc, so I am guessing he can rule out a headgasket? Also, there is not a anti-freeze smell from the exhaust.

I am guessing one possibility could be where the heater core or something was replaced before and some of the smell remains? I know that when we had to have the heater core replaced in my mothers 1997 Oldsmobile, that the anti-freeze smell lingered for a long time when using the heat. For the entire winter if I recall.

I looked at the engine and its clean, but not like it has just been cleaned. There is no leakage from the valve cover, but toward the front part of the engine (below the valve cover) it has a damp look to it part of the way down. When I touched it, it was just like a oil residue.

I am wondering, when the headgaskets fail on these M103 engines, what are the warning signs? I have a M103 in my car and would not mind a heads-up on this myself.

Thanks.