The intruding transmissions were discovered during tests of signal strength and access point positioning as the networks were set up, said HMV group systems audit manager Derek Walklate.
"In some stores we detected three or four strong signals which we could have accessed," he said. The networks were transmitting without basic security measures enabled, he added.
The discovery highlights the importance of basic security on corporate wireless networks and the danger of unauthorised networks being deployed by staff.
Ian Keene, vice-president for research at Gartner, said businesses need a firm, well-articulated policy on wireless Lan, especially because IT departments are finding such cases.
"Most fixed Lans rely on being transmitted along a piece of wire for security. Businesses need to have a policy on wireless Lan, rather than just basic security measures. Although managers often ignore employees deploying individual wireless Lans, that will give free access to a fixed Lan."