Android's new on-body detection system uses the accelerometer in the phone to detect when it's being held or carried by a person.
If enabled, the feature requires a passcode the first time the phone is accessed but then keeps the device unlocked until it is placed down. This means that if you are walking down the street won't have to unlock their phone every time they take it out of their pocket.
The feature doesn't appear to have been announced by Google, but it began appearing in some phones on Friday.
Like the other elements of smart lock, it should be used with caution as it can't detect who is carrying the phone.
"If you unlock your device and hand it to someone else, your device also stays unlocked as long as the other person continues to hold or carry it," reads a message displayed on phones with the new feature.
The smart lock feature was introduced with Android 5.0 KitKat and allows users to set zones around trusted places, such as a home or office, and WiFi or Bluetooth devices, such as a computer or car radio. When the phone is in those zones it will remain unlocked once it's been unlocked the first time.
It can also recognise faces and remain unlocked when it sees a trusted face.
Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is [email protected]