Oddly enough I've had a similiar situation where I was unable to holster a weapon with space and previously submitted it as an error. I was only able to drop the item after relogging and it proved to be a method of duping. You can check that post as well for more information.

Regarding this post: The weapon could have reappeared on the ground once I swapped the backpacks but I was too far from the original point of change from where it happened. I traveled a good distance before noticing the ability to equip the weapon/it vanished.