Thanks Gustavo! as always you're of great help!
Although your solution (I think) only works when the request has a different structure than expected, for instance it's expecting two parameters and only one would be received. I was looking for a not found scenario, where a request would have proper structure but the values entered were not considered.
I think I've found the solution by first selecting the method but no selecting a scenario right away, that will give you access to a "non considered scenario", like this:
if you select a scenario, like the first one:
you'll notice it's a different response! and the first one will show every time a scenario is not found. Like Add operation with 1 and 7 as parameters scenario. I've tried this using SOAP UI:
Thank again Gustavo!
Kind regards
Patricio