At best, this could be a hint that a bit of metal was found in the shot officer that they hope to match.
Possibly some residue, and it could be matched to a particular lot of ammo. There shouldn’t be a lot of spent rounds in a residential neighborhood. My problem would be in connecting the slug with wound - could it be a round fired from a suspect weapon and “found” by a concerned citizen?
The idea being, to plant rounds fired from the confiscated guns in the area, to "keep" them confiscated, I suppose.