With Android 5.1 Lollipop, Google introduced the Factory Reset Protection feature in Android. The feature makes sure that an Android device after being factory reset will not work until you log in with your Google account with which you had signed in to handset previously.
Factory Reset Protection is a great anti-theft tool and ensures that if your phone is ever stolen, it is of little to no use to the thief. However, as it turns out, bypassing the Factory Reset Protection feature in Android is pretty easy. The whole process takes just over a couple of minutes and basically involves jumping through a bunch of menus to get access to the Settings menu at which point one can once again do a factory reset on the device to bypass Factory Reset Protection. The only pre-requisite to this hack is that the phone must have a SIM card installed.
As shown in the video above by RootJunky, the method works on the Nexus 6P as well as other Nexus devices like the Nexus 9 and the Nexus 5X. Android smartphones from other OEMs like LG and Samsung are also not secure from this hack.
Now, the chances of a thief actually knowing how to bypass the Factory Reset Protection feature of an Android device is going to be very likely. Nonetheless, that is no excuse for such a critical feature to have this kind of a loophole.