What is Preferred Network Offload (PNO) mode?

We take the security of our users’ location data very seriously and we’re always happy to be made aware of potential issues ahead of time. Since changes to this behavior would potentially affect user connectivity to hidden access points, we are still investigating what changes are appropriate for a future release.

How to Prevent this?

This means if a users works on the ABC company (as example) and users use the company's WiFi where WiFi name may be named as Company name then it is easy for someone where you have been. If anyone checks the list of last few connection then anyone can tracks your travel, and many of us don't want these.As the EFF team have reported the issue to Google team and Google thanks EFF to make aware the issue with the following statement-As many of us don't want to share our location publicly as I didn't like. So on this regards, EFF have also guide the users how to stop this issue.EFF says- Users who want to protect their privacy right now: go into your phone’s “Advanced Wi-Fi” settings and set the “Keep Wi-Fi on during sleep” option to “Never”.Source:- TNW