what is the ingredient that makes my takeout lo mein smoky tasting?

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:

What Is Wok Hai And How Do I Get It In My Food?