To save on battery, portable devices tend to actively probe for networks (i.e. it is cheaper to transmit a packet than to leave the receiver on for long enough

To save on battery, portable devices tend to actively probe for networks (i.e. it is cheaper to transmit a packet than to leave the receiver on for long enough to hear beacon frames from the access points). Thus, even if your iThing doesn't automatically join the networks out there in a store, it is still emitting packets which contain its MAC address.