Voldemort knew other people knew about the room, he must have, considering it is written in the book the history of Hogwarts, which Hermione points out. He knew this, and so used the room to hide the horcrux. Like someone said earlier, how many people are going to accidentally find the room of requirement, accidentally find the diadem and accidentally destroy it? Not many, I'm guessing.

Also, as Cho points out, the diadem has been lost for centuries, and no living person has seen it (which isn't technically correct, as Voldemort had seen it, but anyway...). This means that even if someone saw it, they wouldn't think it was the lost diadem.

The 'truck load' of stuff in the room may not have necessarily been hidden by students. Someone needs a place to hide their stuff, you are hardly going to be shown a plain room, with nothing to hide the object in. All the other items have probably been created by the room to help the student hide the item.

Why did Voldemort assume that no-one knew about Room of Hidden Things?
I am working on a project based on a lidar data set (for use in LidarViewer in Linux, a basic x,y,z viewer) that was downloaded from #1 below at Opentopo.org and looking to add onto this data set with a shapefile based layer and a gpx based layer. 

In ArcMap 10.0 I am converting the shapefile (road perimeter around lake) into points, then Defining the Projection in Arc Toolbox's Projections and Transformation.  Since Opentopo stated UTM Zone 10, I set that to the roads shapefile, exported, completed my .shp to .las(lidar file) conversion and sent it to LidarViewer.  In LidarViewer my LiDAR data (only a section of the lake's perimeter) is floating the in the middle of the lake (with perspecitve to the road points).  

So, I switched the Tahoe to the original NAD27 in ArcMap, went through the process again with no change in position. Do I have to have a shapefile based on NAD83 for this to work, or is there another way?

Lidar viewer imports .las or .txt/.csv files and only reads a x,y,z no coordinate system whatsoever. 

The other related question is with the .gpx data that I am downloading from the #3 below, it is a .gpx file of a bike path around the perimeter of the lake. I am using the following link to convert .gpx into a text file (to process and import into Lidar Viewer) and the site provides the ability to output long/lat plus UTM coordinates (the output included both zone 10 and 11 South ) This .gpx file also does not project correctly. I was thinking that I should possible try another converter?  

gpsvisualizer .com  convert_format=gpx

Any input regarding the above two questions are appreciated, I can also further clarify any issues that may not seem clear. 

Thanks!

1) opentopography-org -> Data -> South Lake Tahoe, Nevada

Horizontal Coordinates: UTM Zone 10 N NAD83 Meters [EPSG: 26910]  -  Vertical Coordinates: NAVD88 

2) tahoe usgs gov/DLG   UTM NAD27 zone 10 projection

3) bicycling. trimbleoutdoors. com/ViewTrip.aspx?tripID=153934
