Why did Voldemort assume that no-one knew about Room of Hidden Things?
Why did Tom Riddle think he was the only person to have ever discovered the Room of Requirement, when it was clearly full of generations worth of items?
Regarding the Room of Requirement:
Tom Riddle, who confided in no one and operated alone, might have been arrogant enough to assume that he, and only he, had penetrated the deepest mysteries of Hogwarts Castle. Of course, Dumbledore and Flitwick, those model pupils, had never set foot in that particular place, but he, Harry, had strayed off the beaten track in his time at school – here at last was a secret he and Voldemort knew, that Dumbledore had never discovered –
Deathly Hallows - pages 498-499 - UK Hardcover - chapter 31, The Battle of Hogwarts
The furore of the battle died the moment they crossed the threshold and closed the door behind them: all was silent. They were in a place the size of a cathedral with the appearance of a city, its towering walls built of objects hidden by thousands of long-gone students.
‘And he never realised anyone could get in?’ said Ron, his voice echoing in the silence.
‘He thought he was the only one,’ said Harry.
Deathly Hallows - page 504 - UK Hardcover - chapter 31, The Battle of Hogwarts
[Harry] was standing in a room the size of a large cathedral, whose high windows were sending shafts of light down upon what looked like a city with towering walls, built of what Harry knew must be objects hidden by generations of Hogwarts inhabitants.
Half-Blood Prince - page 492 - UK Hardcover - chapter 24, Sectumsempra
I'm guessing the Room of Requirement was not empty when Tom Riddle found it for the first time, judging by the amount of stuff contained in the room by the time Harry's era came about. Assuming Tom Riddle was not a prolific hoarder on top of being the Dark-Lord-in-training, why on earth would he believe no one else had ever been in the Room of Requirement?
The Room of Requirement becomes what a person needs -- could it have been that the room manifested for Tom Riddle as empty?