This only applies to small weapons, like the hammer or wrench, that need to be placed within the inventory [rather than an axe for example].

The bug is that if your character is holding the small weapon and you remove or chance the place that it was stored [backpack, hoodie etc] then the weapon will remain in the persons hand but not appear in the inventory, or the hotslots (the icon will turn white as if it were removed). It will also still appear as an item in the inventory that was removed from the player that originally held the small item.

It's not a massive bug or anything.