Have a great place for Chinese takeout. Love their Lo Mein. It has a great smoky taste. Alas when reheating left overs, the smoky taste is gone. Would like to replace/add to the Lo Mein, but not sure what the seasoning is. Can it be toasted sesame or something else. It really is a great enhancement to the dish. When asked, the Chinese restaurant said they make their own Lo Mein sauce...no help to me.
If I'm understanding your question right, you might be referring to "wok hei," or the "breath of the wok." It's the flavor that restaurant food has because of the high heat of the woks in a professional cooking environment, and home stoves have a very hard time reproducing it. I found it covered in another Cooking Stack Exchange here:
I've seen Chinese restaurants where they cook over an open flame - if yours does the same, the smoky flavor might be from actual smoke, lightly perfuming the food. It can be quite appealing, but if it's an aroma from the restaurant rather than one in the dish, that would explain why it didn't last long enough to be reheated.
There's no easy way to recreate that smoky atmosphere at home without smoke alarms going off, but you might try adding the flavor in other ways - some of the liquid smoke seasoning (usually used to give smoky flavors to meat without the technical difficulty of smoking), which can be tricky to use until you figure out how much you need to get the effect you want. Or smoked salt, which would obviously add salt to the dish.
You might be able to add the smoky flavor to something you wouldn't mind adding to your dish, if you don't want to add extra salt - maybe add a drop of liquid smoke to toasted sesame as you suggest, or find something else altogether... I saw a recipe for smoked egg yolk bottarga, for example.
This video claims it is the mushroom flavored dark soy sauce that causes the "smoky" flavor. I also have been searching the "way" Chinese restaurants and buffets create that smoky flavor I don't get at home.