I think Voldemort's arrogance has to be included in the equation. Voldemort truly believed that he was the only person to discover the Room of Hidden Things/RoR/CaGR, whether that seems logical or not. After escaping Gringotts and jumping off the dragon into the lake, Harry finds himself inside Voldemort's head, able to read Voldemort's thoughts about the Horcruxes:


  [Voldemort] As for the school: he alone knew where in Hogwarts he had stowed the Horcrux, because he alone had plumbed the deepest secrets of that place ...
  And Hogwarts ... but he knew that his Horcrux there was safe, it would be impossible for Potter to enter Hogsmeade without detection, let alone the school. Nevertheless, it would be prudent to alert Snape to the fact that the boy might try to re-enter the castle ...
  Deathly Hallows - Page 444 - British Hardcover
  Harry says, "He thinks the Hogwarts one is safest, because Snape’s there, because it’ll be so hard not to be seen getting in, I think he’ll check that one last, but he could
  still be there within hours –'


I also think Voldemort was emotionally compelled to hide at least one Horcrux in Hogwarts. Hogwarts -- not unlike for Harry -- was the only place Tom Riddle/Voldemort ever considered a home. Given his propensity for attachment to things (Hogwarts, Founders' items, the snake Nagini) rather than people, it makes sense that 1) he would want to hide a Horcrux in the place that he considered home and 2) due to his emotional attachment to Hogwarts, he could have very well thought that the castle itself claimed him as its ultimate master. If so, then of course no one else would have discovered the Room of Hidden Things, because Hogwarts belonged to Voldemort (in his mind) and Voldemort alone. This represents a twisted sense of entitlement, the inability to see others as equals (seeing people as unskilled in achieving the high level of magical power that he himself had), and unable to imagine that any wizard other than himself could possibly discover or work out the secrets contained within the castle. So, to answer the question, the reason Voldemort made such a moronic mistake in leaving the diadem Horcrux essentially in plain view is because his thinking is distorted and skewed -- for he recognises no other as an intellectual equal or someone to learn from (he already believes he knows it all -- and is full of logic and thinking errors.) Once again, Voldemort underestimates that which he does not value, and there is so much that he does not value that his perception of the world is myopic, imbalanced, and not reflective of reality.  

Why did Voldemort assume that no-one knew about Room of Hidden Things?
Based on this question Avoiding crew fatigue on empty leg flights I learned that there are "empty leg" flights or "ferry flights".

These seem a good option but I would like to know more about them.

Where are these announced? How can one travel this way?
