Sorry, I’m not buying it; sounds more like a shakedown to me.
Didn’t the officers check the guy for residue from the gun? That would seem like the most important thing to do. That would have shown whether or not the guy had even handled a gun.
And, the story about being pistol-whipped (was that supposed to be the arresting officers who did that?).
The whole thing stinks.
That should have been done. I have not heard anything about that, so I don't know if it was or not.
The test checks for the presence of nitrates. It will not show positive if the gun was just handled. It would show positive if the body parts checked (normally the hands) were near a gun when it was fired. He would not necessarily have to be holding the gun when it was fired.
False positives are possible. For instance, if you had been handling or spreading fertilizer, your hands would then test positive for nitrates.
The residue can be washed off, but I am not sure how hard that is to do. It may take more than one washing.
Over time, skin sloughs away. So anything on the surface of the skin, whether soluble or not, will be shed in a few days.
The caught him 2 days later and i’m pretty sure unless it’s really blatant residue is checked for in a lab and not by the officer at that point.