[Kristine1975]

The (partial?) source code for Thief 2, SS2 etc. was leaked a few years ago. You can still find it on the Internet.

[steinuil]

I know, but using it is not exactly legal and having to rewrite it all from scratch would be bothersome, even using the original code as reference. There's a set of C++ macros used for "scripting" the AI and the missions which I would immediately tear out if I had to rewrite the engine, and I'm afraid replacing it with something more sane while still retaining compatibility with all the fan missions and so on would take a lot of time. Plus, as you said, not all the code was in the leak (though I don't know exactly how much is missing).