Found this a few days ago. Sorry if it's already been posted
I was in free mode with my brother, and we decided to take a chopper to Playboy X's house. (Cos we can't get in through the doors)
After getting inside the house, I looked at the TV and noticed the buttons from the multiplayer lobby were in the bottom right of the screen.
I told my brother to look at it, but he couldn't see it, so I think it's a 'host only' secret.
Heres a pic
Crappy quality, but still proof that it exists.