No, it's not. So, yeah, I would assume the backpack has been there for a while, but I don't know how your police department conducted their ground search, so I don't want to criticize. But if a murder weapon is somewhere within four blocks of the crime scene, well, I would suggest that a good expanding area search, block by block, should have recovered the murder weapon sooner. I've done those kind of searches, and sure, you miss stuff. You look in yards, under bushes, in trees, on rooftops, and you could miss something in a heartbeat.
Cops are human.
Then a jogger or a biker comes along and finds something you missed. That's quite often how we turn cases. You exhaust your leads, and something just turns up weeks, months, and sometimes years later. It's not like TV, you know.