I had a dog that kept disappearing, there was one spot in the kitchen where he would walk to and suddenly vanish and show as off the lot. If I caught it quickly I could click the whistle to get him to reappear but if not then he'd be gone until I reloaded the lot. I finally found one thing that seemed to fix it! Next time he vanished, I went to the owner's "Relationships" panel in the lower right, found the dog there under pets, and clicked "Invite To Lot". My Sim uh.... called the dog on his cell phone and invited him over :D and he came back, and since then, hasn't vanished at all, and it's been multiple days. Maybe the dog was just mad that no one would check out his cool new dog phone?