I figured out a really good way to help me explore tombs. If you buy the Eye of Horus while you're in Egypt, you can use it anywhere. Make sure you have it activated while you're traveling and then when you go into a tomb, it will cause all the treasures in the tomb to light up blue. Then if you think you're done exploring a tomb and you can't seem to find anymore secret doors, look around at all the blue lights. It will help point you in the right direction and if you see that there are blue lights in the dark outside of what you've already explored, then you know there are more rooms that you need to find. Get close to where the blue lights are and run your cursor across all the walls and chances are, you will find a hidden door that you didn't know was there. If you can't find any hidden doors, then there's probably a hidden staircase that you need to find. Be sure and run your cursor over all the wall torches since some of them act as levers and be sure to mourn at any tombstones and even use toilets! You never know what will open the secret doors or staircases! It took me forever to get through the Landgraab mansion and it was because I didn't realize that flushing one of the toilets turned off some of the floor traps! Also in the Landgraab mansion, you have to turn on the stereo in the library (I think that's where it is) to activate something.....can't remember if it's the floor traps or a hidden door.
Bottom line......if you think you're stuck.....try clicking on EVERYTHING!