Proven, Su Yang's speculation was correct.
Before he planned to enter the Edge of the World Dungeon, Su Yang had already started planning. He wanted to create an illusion that he had been in the dungeon for at least two days. Therefore, a day in advance, the third young master instructed the kitchen to send an extra meal to the dungeon.
And this step was indeed done right by Su Yang. When Ge Kaiyang started investigating, he discovered that the kitchen had been sending an extra meal to the dungeon two days in advance, leading him to suspect that Su Yang had been sent to the dungeon two days ago.
And for those two days, the person sending meals to the dungeon had always been the third young master. So naturally, Ge Kaiyang had to find the third young master to inquire about the true situation in the dungeon.