Condensation? Do you live in a humid environment? How frequently does this happen and how frequently do you drive the car?
In the 280s, there is a water jacket that encircles the intake which, I guess, is intended to prevent intake icing. Early versions were copper pipe around the outside of an aluminum sleeve. It would be darn near impossible for that arrangement to leak into the intake. Later models were cast into the aluminum. Still, an unlikely leak. In either case, it wouldn't seem logical that the water would migrate against the air flow into the duct.
If the water contains anti-freeze, it should have a mild odor sort of like maple syrup. That may be hard to detect over the smell of rubber from the duct.
Ray
'68 280SL 4-spd Coupe