If you're not watching the magic gnomes they will teleport around the property (generally at night when everyone is asleep, but also at night if sims are up as long as no one is in the room with them or paying them any attention). Usually into a room with a radio or tv, or where they can be the most annoying: like next to your sim's bed so they can't get out of bed in the morning. Its also not a good idea to have gnomes when you have done the "wire house for sound" upgrade on your stereo, as they will turn it on in the middle of the night and wake everyone up!
Sent from my BlackBerry 9720 using Tapatalk