Many who piggyback say the practice does not feel like theft because it does not seem to take anything away from anyone. One occasional piggybacker recently compared it to "reading the newspaper over someone's shoulder."

Piggybacking, makers of wireless routers say, is increasingly an issue for people who live in densely populated areas like New York City or Chicago, or for anyone clustered in apartment buildings in which Wi-Fi radio waves, with an average range of about 200 feet, can easily bleed through walls, floors and ceilings.

Until now, using other people’s wi-fi was considered more cheeky than criminal. With wi-fi operating at speeds up to 20 times faster than normal connections, a piggybacker is unlikely to slow down a user’s system unless they download huge files. But there are growing concerns about the ease with which networks can be accessed. In the US last year, Brian Salcedo, 21, was sentenced to nine years in prison for siphoning credit card numbers over a wireless network from a hardware store.

Although the genesis of warchalking arose from the Internet-inspired "free information for all" concept—with owners of short-range 802.11b or Wi-Fi standards alerting those in need of access points—the idea could take on very sinister forms if your company is warchalked and nonregistered users use your network to get on the Internet. Cell phone manufacturer Nokia has termed warchalking theft. And it's not in good stead with the FBI either, which warns that unscrupulous piggybackers cannot only slow your systems down, they could use your network to issue spam.