The missing case in between Egypt and Rome would have taken place in Ancient Greece.
It seems like a logical place to put one, especially if (as speculated) it involved Polly Tix as your guide. Maybe you could have met Aristotle or another such philosopher. They might have taken it out because it was too similar to Rome.- Most likely it would have been about the birth of Athenian Democracy, which fits with Polly's MO, and would feature some code you'd need to decipher in order to catch Dee Crypton.
