The admission comes after the data protection authority in Hamburg, Germany, called for an audit of the WiFi data recorded by Google's Street View cars.
In April, Google said its Street View cars collected only publicly broadcast WiFi network names and MAC addresses from WiFi routers for use by location-based products such as Google Maps for mobile, which enables people to find local restaurants or get directions.
The audit revealed that data being transmitted via open WiFi networks was also collected, which Google now says was unintentional.
According to Alan Eustace, senior vice-president of research and engineering at Google, some experimental code for sampling all categories of public broadcast WiFi data was included by mistake in the software used by the Street View cars.
"The project leaders did not want, and had no intention of using, payload data", he said in a blog post.
