Recently I was crafting inside the home of Wilford in Brittany Alleys. I was in Friends mode so he surely did not see me because he decided to unclaim his house lot, switching from a taxed deed to a tax free. When he unclaimed his lot I was ejected out of his house and put directly next to his house sign in the perfect location to claim his lot from him.
Single Player Online/Friends mode could be more effectively exploited against home sales and transfers because the location of the home and a general time of sale can be witnessed in the marketplace forum. After a sale in the forums someone wanting to steal a lot could go into Single Player Online/Friends mode go to the sold lot and then run a lot claim macro to swipe the lot the moment it is unclaimed.
Players in Single Player Online/Friends mode do not show up when pressing Num+0 to view nearby players, they are completely invisible with no way to detect them.