Hi,
It could be a faulty Power button.
You would need the schematics to be sure, but here's the maintenance and service guide for the laptop. Go to p.79 to view the procedure to remove/replace the power button board.
I don't know the actual pinout connections for the power button for the cable that connects it to the motherboard, but you should be able to work it out with an Ohmmeter once the cable is disconnected from the board by operating the button and checking for a short circuit between two of the pins which disappears when the button is released. If there is none at all between any of the pins then the button is faulty.
If the button is faulty the part number for the button board is found at the top of p.79. Search online using the part number only to find suppliers that suit you.