After killing that mummy, i cannot for the life of me find the button. where on the wall is the button? is this a bug that i cannot find it?

The button is in the elevator...but get out as soon as you push it and ride the top of the elevator up to the secret room above.

BTW, the best way to find buttons and other items that can be hidden in the environment is to cast the "Spirit's Eye" spell which will show these things as a little white dot on your map.