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
back in the late 60's, early 70's my mother was a waitress in an authentic chinese where the owners came from Hong Kong when i was young.. we had a little jar of browning sauce for the rice.. i believe it was made by La Choy. it was very thick, dark, and gooey. you only needed to put in a very little bit in the wok to a bunch of rice.
Not authentic, but a cap full of Kitchen Bouquet works well for me. Place the cold cooked rice in a zip lock, add the soy and oyster sauce, add a cap full of Kitchen Bouquet and mix well, this way you can squeeze out any lumps. This now becomes a rich brown rice ready for frying with the other ingredients. The cooked rice was 1 3/4 cup of dry jasmine rice. The cap of KB is about 1t., less can be used.
A Chinese woman who works at a restaurant supply place put me on to "Thick Sauce" It's a molasses-like sauce, which gives the rice a rich brown color. A little goes a long way. It can be found in oriental stores.
I suspect it is Maggi's seasoning sauce, which you find in all oriental markets. Joyce Chen's cookbook says in China, you NEVER add soy sauce to make fried rice. She refers to a "brown sauce".
Most likely to be some form of Dark Soy Sauce, especially if it was a thin liquid. A few drops would have changed the colour of the rice substantially. (Of course a Dark Soy Sauce diluted with water would help control the amount of colour change desired.)
Oyster sauce or black bean sauce.
It could just be Soy Sauce mixed with Cornstarch. That adds a nice sheen to the rice.