if you wanted to be a cheater then you could try the testingcheatsenable true, shift click on the tomb, go into a buy/build mode, which would revel all the passage ways, and secret passages. This would show you where everything is and possibly the room you missed with the specific symbol you are looking for.
But that's the big cheater way.
Without the cheat I would try just testing all the walls for secret passages. I also look for 'empty' spaces, spaces that I might have missed, spaces that are still blacked out where I've yet to explore.
good luck!