On some sliding doors the lock is fitted upside down so the hook swings upwards when the door is locked to prevent the door from being lifted off its rails.
There are also aftermarket devices which you fix to the bottom and or top of the door and they have a bolt which pushes into the door itself and locks in place with a key. You have to drill a hole about 10mm diameter in the door for the bolt to fit into. Available from most Locksmiths