All stainless steels are equal, but some are more equal than others! There are standard "recipes" for making any given stainless steel
(302, 316, etc). Each recipe has certain tolerances for the amount of each ingredient.
For example, the recipe for 316 stainless is:
The variations can make a difference to both the material's rust resistance and to its "megneticness". Just because a piece of steel is magnetic, it doesn't mean it is not stainless. Also, just because a piece of steel displays rust, doesn't mean it isn't stainless. I have, through my job, worked with various stainless steels, sourced from various parts
of the world. 316 sourced from West Germany
is a different beast from 316 sourced from Korea
Having waffled on iterminably, I will just say this: Hose clamps, rusty or not, should be inspected regularly and should be changed periodically regardless of their perceived condition.