Residents searching for the slug?
This brings to mind a few problems - chain of custody and proving it was the slug that hit the officer.
Oh, let them stupid themselves into oblivion. Then maybe the public will wake up and demand Cops with a Clue (TM).
At best, this could be a hint that a bit of metal was found in the shot officer that they hope to match.
On the other hand, fishing expedition? As various bits of slugs and bullets get handed in, if it even looks slightly like it came from a now verboten gun, raids will follow on the homes of the very people that found them.