From the Beeb article:
While the chance that someone will piggyback on your wi-fi network is low, there have been cases in the UK where this has happened.
In a block of flats I would think quite the reverse. One has heard of people (without malicious intent) using others WiFi internet access.
It is astonishing how many users permit their router to broadcast its SSID and also fail to restrict it to authorised MAC numbers (which I believe is also known as Physical Address).