Easter egg: On the Deleted Scenes on Disk 2, highlight "Pursuit at Rushmore: The Unseen Chapter" and hit left and then down. Hit enter to see an easter egg featuring Nicolas Cage and the director.
Easter egg: On the 2-disc Collectors Edition, go to the second page of the "Bonus Features" and highlight "Evolution of a Golden City". Press the left arrow button on the remote twice and a small figurehead will appear on the lower left corner of the doorway. Press the "Enter" button and enjoy.
Easter egg: On the 2-disc Collectors Edition, on the first page of the "Bonus Features" highlight "Street Stunts: Creating The London Chase". Press the left arrow button and then the up arrow button and the eyes on the door will light up. Press the "Enter" button and enjoy.
Easter egg: On the 2-disc Collector's Edition, on the first page of the "Bonus Features" highlight "The Book Of Secrets On Location". Press the right arrow button, then the up arrow button twice on the remote and a small figurehead appear on the lower right hand side of the doorway. Press the "Enter" button and enjoy.
Answer: It is never explained what was on the page. It was just a cliffhanger to set up for a 3rd movie, I think.