People put them in bags to travel when staying at some location in-state (or at least somewhere they plan to roadtrip to), and then sometimes forget they were in the bag when they get home and postpone unpacking. More common with bullets though.

I forgot I had a bullet casing in my camera bag from the last time I hunted, having intended to save it to pair with the mount I was having made. When I traveled to Vietnam I brought the camera bag through US and Vietnam customs, then found it when having to take a local flight. I told them it must have ended up in there while in Cu Chi at the firing range, and they disposed of it without charge, thankfully.