After the scale puzzle and before the cobra puzzle, we're to return to tunnel three and move the coffin cover to see a mummy inside. When I get there the cover is off the coffin and no mummy. Did I miss something somewhere? The mummy is mentioned in the conversation with Jon Boyle later.
It might not be triggered yet. But if you already talked to Jon about finding a mummy - no problem then. Continue on or you can look again to see if it is there. Have fun.
The egg does not have anything to do with the gameplay - they are optional.
Ah - so in the talk with Jon; the mummy is still not there. Then continue on and then later go back to see if you find a mummy. It might just be time between the cat room and finding the mummy.