Originally posted by Ataxio:

It's not limited to the entrance of caves, any item dropped outside the influence of a worktable will vanish, bare that in mind. AFAIK if you DO want to preserve dropped items anywhere, just make a worktable real quick and place it nearby. Obv thats not possible in building restricted areas.

you can build a campfire though which should keep items from despawning in its vicinity in a cave/interior