Whats that? Me? Cheating... Never its clearly an X marking the spot where the treasure is. Whether its just a symbol on the map or an actual giant X on the ground it is a stereotypical marking for where treasure is hidden. Now how can we subvert this so it can be slipped in past the players ever watchful eyes?
First idea I had was make the room be in the shape of an "X" or maybe have it be where two hallways meet. If using a room this may be seen through quite easily as you are putting a giant X on the map, literally. With the hallways if your careful and make it so its the only place where 4 hallways meet like that you may have it slip by without the players noticing right away. Both of these are interesting ways to include treasure without just giving it away.
Next I tried to think of a more metaphysical. The first thing I came up was a magic treasure chest that opens if you think of the letter "X". This of course is an interesting idea, a treasure chest that only opens if you think of the correct thing. Besides just using X you could have a riddle written on it and have it open when they think of the answer or any kind of thing like that.
I followed that idea up with the thought of what if there was a magic door that went to different areas depending on the first letter of the last word spoken aloud. This would mean having to have a number of different rooms but on the bright side you can really mess with the players till they figure it out. Also you don't need 26 rooms you can have a couple letters go to the same place. Of course because of the theme obviously to go to the treasure room you need to say a word starting with "X" which oddly enough the word X does not.
You could also include a game of tic-tac-toe. The secret would be instead of having to win all that needs to happen is have the X's win. Give the players a choice of X's or O's and if they choose O's they may end up quite frustrated with the fact they are winning yet not getting anywhere.
For my final thoughts is on a treasure map using "X"s to mark instead of treasure but danger. This could throw the players for a loop. This is the biggest subversion of the theme I could thing up as it takes the expectation of treasure and instead gives danger.