Though it hasn’t widely been used, Warchalking is a method for letting people know that Wifi is present. This might explain any strange symbols you’ve seen on the street.

In Warchalking an open circle means closed node

A circle with a W circle in the middle means WEP Node

Two C’s point Opposite each other )( means open node.

Cars could drive around with their laptop marking these symbols on sidewalk, streets, or any other visile place.  This would alert passerbys and neighbors that there is a connection point.  Of course now even with a wifi enabled cellphone it isn’t too difficult to find an open connection.