Yup - that is a possibility. But don't think for a moment that if a thief wants something bad enough that a hard case will keep it safe.
I have a hard Givi 55 ltr trunk and put my more expensive stuff in there - but again I know there is a possibility that it could be missing when I return.
My soft cases are more for drinks and snacks etc Maybe a few groceries - but I'm usually on my way home after getting groceries.
Like most things there are Pros and Cons to each.