Amusingly, a lot of source was found in an old Dreamcast dev kit: https://www.rockpapershotgun.com/2010/12/14/shipping-receivi...
Would love to see Dark Engine games get the treatment Quake games got, or something like OpenMW
It would also help me forget the crushing disappointment I felt with the Thief games since 2 (with the exception of the Shalebridge Cradle level in Thief 3).
I thought it was some random user called Le Corbeau from the french forum Ariane4ever?
The dreamcast code mentioned by jeffbax is the current assumption of how he got his hands on it.
We are only starting to see some games like MGS5 surpassing the AI of Thief but most of them try to combine stealth with power fantasy and that's a bad mix.
Thief had it right with its underpowered character lost in a maze like level.
The Thief games still have a very active community that regularly produces high quality fan missions, and one of the original engine developers has started releasing unofficial patches to make the games run on modern systems a few years ago. I'd love to read the games' full source code and to be able to play them on Linux without bothering with Wine