Based on reports starting to be made by users, Google is either testing or slowly rolling out a new lock mode for Android devices designed to detect when a device is physically in a user’s possession. The lock mode, called “On Body Detection” uses a device’s sensors to detect whether a device is being held in a person’s hand or is in their pocket and will keep the device unlocked. If the device is set on a table or something similar, the device will lock, requiring a user to employ their normal unlock method.
The new method appears to have been designed to help thwart thieves in case a user accidentally forgets their device somewhere. One thing users have determined is that the on body detection feature will not lock a device if the device is handed to someone else, so it is not tied to a specific user – only movement typically of being on a person’s body or in their hand.
Google has not announced anything about the mode, but it appears to be included in the most recent recent Play Services update. Thus far, only users with devices running at least Android 5.0 or higher have the new mode available, so it looks like it may be a Lollipop-only mode.
source: Android Police