Hey Sarah,
I love this sauce too. As far as a secret ingredient, there's really not one. It contains the typical makings of any hot oil or chili sauce. I do agree thought that it's got something more. I think the flavor we're picking up on is what's sometimes known as "wok hay." It's that extra smoky taste that you get from a really well seasoned and correctly used wok. Tibetan hot sauce is slightly different in that it's produced using extra hot oil. It wasn't practical for mountain travellers to slow cook garlic and chilies over a long-burning flame because there just wasn't enough wood. So, instead of low and slow, they used an extremely hot flame. When the garlic, chiles, and onions are added, they instantly caramelize. I hope this helps a little.