I came across an interesting article in the Tech News section of my local paper and after a quick google search found an article relating to the new craze with hackers ...

'War chalking': New hacking threat to firms?

Business leaders are warning a new craze called "war chalking" can leave firms open to hackers.

It involves drawing symbols in chalk on walls and pavements to mark points where signals from nearby office networks can be tapped into.

Anyone with a suitably-equipped laptop can then switch on and surf the web ......

Full article here

If you are running a wireless network for your business it might pay to keep an eye on the externals of your building for any strange symbols etc ... that may indicate a breach in your security.