What is the black, slightly sweet, flavour in some Asian food?
Hoisin sauce, oyster sauce, and black bean paste seem to be the most likely culprits.  What is the texture of the sauce? If it's smooth, it's hoisin; but hoisin tends to be salty. If it's not smooth, it's black bean paste. Oyster sauce is a little oilier than the other two, and is used in almost every Asian sautee.