I feel the urge to make a joke about Necromancers for some reason, but I can't put a decent one together. Four years dead, little Timmy, and you doth rise again.
Familiars are WYSIWYG. They aren't there, unless they're there. If they're claimed to be elsewhere, it must be on the person. I've seen snakes played that way before, like Shadow Asps, where they are within the jacket/robes/coat of a person, but for things that aren't a coiling serpent, there's really no realistic way to go about it. A Hellhound, Beholder, or otherwise do not have a way to be. Even having a set of Bat bones in your inventory would not constitute the correct rules, as summoning a familiar is summoning them to you, not re-animating a set of bones constantly.
It's among the mechanics not worth overthinking too much.