OK.....Just a thought. Dobermans, even those with "cast iron" stomachs are known to vomit occasionally.
There are a variety of reasons. Bile accumulation on an empty stomach, food sensitivity, exercise to close to feeding,excessive water consumption.... The list goes on. Given that your basically healthy boy vomits only occasionally, I would suspect ingestion of non-nutritive/non-digestible substances when he is out of your view. It can be grass, sticks, mud, cloth.... you name it. When taken to the extreme, it is called Pica. Pica Behaviour in the Adult Dog
This is a very common situation with our breed.
Now, as I said, this is just a a thought. But, my main comment would be.... If this is an only an occasional experience, with no apparent ramifications, and you have had a professional review of the situation, I would not be overly concerned
My one caveat would be that if you see an increase in vomiting or an obvious change in your dog's behavior (especially vomiting, appetite or bowel activity) a vet visit is required.
I am not a vet,
My comments are simply based on my years loving this breed.