Fired cases are not ammunition, unless so defined in law (and while apparently that has been done in some places, its quite rare). And actual arrests and prosecutions for having a fired case alone are more then rare.
Even the most rabid anti gun areas generally allow their officers some latitude, particularly when it comes to a single fired case or two. Even if the local law provides for it, I doubt any prosecutor would try such a case. Cost vs benefit to the public, and the tremendous public opinion backlash that would result from it just make it a waste of their time.
That being said, in a situation where a fired case is something unusual, it could, and perhaps ought to be grounds for suspicion. Grounds for an arrest, on the fired case(s) alone? No. But seeing fired brass where you usually don't see it ought to have officers wondering what else is going on.
A few spent shells in the back of a pickup truck out in the country is one thing. Spent 9mm cases on the rear seat floorboard of a sedan, occupied by inner city youth, is quite another.
All else being equal (and it almost never is) bigger bullets tend to work better.