Non-shivering thermogenesis in adults is due to
Here, I would like to clarify some important facts which you must not be knowing and these have not given in our standard textbooks also.
Once we hear the term "non-shivering thermogenesis" the idea immediately comes to our mind is that non-shivering thermogenesis means generation (production) of heat that is not due to shivering.
This idea is not wrong, but the concept of non-shivering thermogenesis is slightly different.
Shivering thermogenesis means when there is the production of heat inside the body and there is shivering also. This heat production due to the contractile activity of muscle (in shivering) and may be associated with heat production due to increased metabolism in other organs like liver → whenever there is the production of heat with shivering (whether the heat production is due to shivering or other metabolic response), it is called shivering thermogenesis. In simple words, the production of heat (whatever the mechanism), which has to shiver with it is called shivering thermogenesis.
On the other hand, when there is the production of heat without any shivering, it is called non-shivering thermogenesis.
In humans, a neonate can adjust to cold exposure without any shivering, i.e. non-shivering thermogenesis. Whereas human adults always respond to cold with shivering, and there may be activation of another thermogenic mechanism also → shivering thermogenesis.
Now, you must be thinking that if we will wear many woollens than we will not have to shiver to the cold exposure. Yes, you are right, but in that case, we are preventing heat loss (conserving heat) but are not producing heat (no thermogenesis).
If an adult man is suddenly exposed to cold, without wearing woollens or too many clothes, the response will always be shivering. On the other hand, neonate can generate heat without shivering.
I think I was quite clear in my discussion and now you know what exact shivering thermogenesis and non-shivering thermogenesis mean.
In human adult, there is no non-shivering thermogenesis as there is no brown fat in the adult:-

"As far as adult man is concerned, there is no evidence that non-shivering thermogenesis occurs".— Energy metabolism in animal and man

Whenever, wherever and in whichever, non-shivering thermogenesis occurs it is due to sympathetic stimulation (norepinephrine).
In adults of lower species, non-shivering thermogenesis occurs in the liver, skeletal muscle and brown fat due to catecholamines (norepinephrine).
Thyroid hormone
Brown fat between the shoulders
Noradrenaline
Muscle metabolism