Scientists make it very clear that Coronavirus were not created in a Laboratory. When a new virus is synthesized, a genetic footprint is left. So it isn`t hard to analyse whether the virus was created by humans or not.
If you were the person or group responsible for creating and spreading a new virus or biological weapon, would you do it in a way that allow other people to track it back to you?
A terrorist group WANT to be found. So it is not a problem for ISIS, for example, to leave the genetic annotation in the virus. They want it because their goal is to create terror, so they will, sooner or later make a CLAIM out of that.
But if a person or a group want to keep it a secret, it is reasonable to think that these people would go for different approaches. A few options come to my mind:
1- A new technique. The group developed a new technique, and since it is going to be used as a weapon, just like any other weapon, they don`t make this information public. Happened several times over history.
In this case, a synthetic virus is created with no genetic footprint.
2- An induced or controlled environment. Instead of creating the virus myself, I use nature for this purpose. Forcibly make people and animals live close and pick the most aggressive virus.
Could be done in “prison-laboratories”, for example. This is more like a Josef Mengele approach. The animals, the patients and the strains are carefully chosen.
The problem with this approach is that the mutations are random, so it will probably take a lot of time to happen (and this is why choosing the correct individuals here make a huge difference).
3- Leak an unknown virus. After identifying a new virus in the population, instead of cataloging and making it public, the agent would plant it in a way that it would spread quickly and efficiently.
In any case, before leaking it, the agent would want to create the vaccine, remedy, or whatever is necessary to not be affected by the virus.