The film, The Water Diviner (2014), opens with the end of the battle. The turks charge the allied trench to find that they have retreated under cover of darkness and rigged it to appear active. The trap pictured is an unmanned rifle with a bucket tied to the trigger that is slowly filling with water. When there is enough water in the bucket, the trigger will pull and the gun will go off.
Were the turks really surprised to discover that they had left? Did the allies slip away unnoticed? Were such traps actually in use? The whole thing seems like artistic license.
In brief: The devices the OP describes were used. They were not intended to be traps and would not have worked to injure the Turks. There were a variety of methods used to time irregularly fired shots during and after the evacuation to conceal that the trenches were no longer occupied. The device described by the OP was considered one of the simplest and was invented by LCPL Scurry of 7th Battalion.
Allied High Command was intensely aware of the vulnerability of an evacuation to an attack on the rear party which could lead to heavy losses.
The allies did slip away unnoticed with the mines (explosives buried in underground tunnels) at the Nek being fired at 3:30 a.m. and the final check that all troops had been evacuated occurring at 4:00 a.m. on the morning of 20th December, and the Turks were not aware of it until after they had departed. The Turks were aware of activity in the period leading up to the evacuation and had discussed it at senior level but had not been able to tell if it was troops being evacuated or reinforcements being landed, and the discharge of mines at the Nek was initially seen as the prelude to an attack by the allies. It was the Turkish counter attack which informed them that there were no allied troops facing them.
The evacuation was the result of a complex logistical plan where even the allied troops and unit commanders were not told that an evacuation was planned until the last few days. They had been told that due to the onset of winter, stores and equipment were being withdrawn to camps on the islands and that only necessary troops were being kept in the front lines so units were being thinned out with soldiers withdrawn. Despite this, by December 12th / 13th rumour among the troops resulting from the preparations was fairly sure of an evacuation and extra security steps were taken to prevent rear echelon gossip reaching the Turks.
Reference: An extremely readable and detailed history of the planning and evacuation, accessing Allied, German and Turkish views is contained in Chapters XXX and XXXI of Volume II of The Official History of Australia in the War of 1914-1918 by C.E.W. Bean.