Privacy of Correspondence

How strange! The only bodies found next to the ship were those of Tep's cultists. One of them had a letter in his pocket that you found... It was signed by Charles de Doucer! This is so suspicious that perhaps the privacy of correspondence may be violated...
Well, that's disappointing. The characters in the letter look familiar, but not the language. There's a plan in the letter, though: some carelessly sketched corridors and arrows. Looks like this is an escape plan indeed... But where from, exactly? This find should be shown to the rebels' leader.

Charles de Doucer? Never heard of him before. Where is he?

So this was in a dead cultist's pocket? I wonder where they escaped from. There's nothing like this plan near the sarcophaguses. It's as suspicious as it gets...