I think it takes more then just entering the house, I'm pretty sure you actually have to do something with it (i.e. eat food that's placed in there, buy/sell something to NPC on your estate, garden, chocobo stables, etc..)

Each time I go to my houses I utilize something in there like from my examples and I've yet to get a message, so gives me time to enjoy Fallout with freaking out about losing my house lol.

Sorry you lost it though, if I am correct though then SE needs to clarify to people what exactly they need to do to keep their estates if entering them alone is not enough.