Hi @aaftab.mughal80 ,
Thanks for bringing the issue to us via this post and email.
Kindly be advised that the Powercore+ should turn off when your phone is charged up, but they will stay on if the phone reaches 100% and the phone is still active, just as @Element321 said, because there is still current draw from the Powercore+ to keep it on.
Since your phone was turned off overnight, and you noticed the LED indicators are still not off, could you confirm if you used your supplied iPhone cable or our Anker lightning cable to charge your iPhone 6S? If yes, then we think it is possible that the powercore+ in your hand is faulty.
Hope this helps. If the problem persists, please feel free to contact us via "email@example.com".