Unprotected modern utility meters
Modern utility meters transmit readings of consumed water, gas and electricity using radio signals. The signals which are transmitted on regular basis are then intercepted by utility company employees as they walk or drive around using receivers. According to a study performed by the University of South Carolina, an attacker can easily tap in using some basic equipment and from the data collected determine whether a house is vacant or not such as, the trend in consumption may indicate that the occupants are away on a holiday!
Read the full article here - http://www.newscientist.com/article/mg21628875.300-wireless-meters-tell-snoopers-when-you-are-not-home.html