There is a lady at work that makes the most awesome and most delicious Chicken Fried Rice on the planet. She uses white rice with some sort of brown sauce that is added when frying the rice in the wok. She won't tell anyone what that brown sauce is other than she is using soy sauce. Not true! It is extremely mild, and slightly brown in color. Does anyone have any idea?
If it really is authentic, then it is soy sauce. Some recipes also use oyster sauce but I would not call that authentic or traditional.
Soy sauce can mean many things; it might just be a different soy sauce from what you're used to. There are light vs. dark soy sauces, and also fermented vs. hydrolyzed kinds. A naturally fermented light soy sauce would probably be (a) mild and (b) slightly brown.
Some other common types of sauce that I've seen used in stir fries are hoisin and satay. But, as with oyster sauce, I wouldn't call those authentic or traditional. That "some sort of brown sauce" is almost certainly soy sauce. It might include XO sauce, although you would notice a seafoody taste in that case.
Not knowing what Chinese style or if influenced by Malaysian Vietnamese cuisine etc, there are a number of possibilities...
As you said slightly brown (meaning not dark?), I will hazard that it is a fermented bean sauce; more savory tangy than salty. They come in many varieties from yellowy to caramely to toasty in color. Thickness much like ketchup which varies in runniness too.
Just a guess