Good point. I’d not thought it through as far as I’d thought. Thanks!
Incidentally, another nice little bit of plot working: I don't think just any house elf could have aparated into the Malfoy's prison; I'm sure they had magical protections on their house. But since Dobby was their servant, they would have wanted him to have been able to aparate in and out so he could do his errands without disturbing them. Guess they forgot to do the magical equivalent of changing the locks after he left.