The party opts not to deal with the sarcophagus, but instead press on. Each prepares for danger, but finds none when the secret door is opened. Instead they see that they are back in the room where they retrieved the urns and and gold bowl. The secret door had a small, hidden grip that allowed you to open it from one side, but it would have been impossible to find a grip that would have allowed you to open it from the other.
It was the secret room that was next to the portal room. It seems you have found the way back out. Of course you still haven't found a way to open the other portals yet, or the black pearl, but you have found the journal you were originally tasked to retrieve.