Werewolf Headquarters

Those pack hangout buildings can be interacted with when your sim becomes a friend of the pack. Becoming a friend of a pack doesn't require your sim to be a werewolf, so even non-werewolves and children can become a friend of a pack. Plus you can become a friend to both of the packs. Once that is done, your sim can use facilities there and hang out there for fun.

Joining does require your sim to be a werewolf, and then the options are expanded a little. And if your sim becomes an alpha, you can even redecorate it.

Oh, and they are rabbit holes.