Teebo and Kai: The Search for Awl
Teebo and Kai: The Search for Awl was an unreleased game by Escape Factory running on the Goldsource engine. The game was not publicised during its development and much of the known media is from developer portfolios and assets which were re-used in Space Quest. The game was commissioned by Valve after they was contracted by Microsoft in 2000 to develop an exclusive game for the then-unreleased Xbox (https://www.unseen64.net/2021/04/09/teebo-kai-xbox-pc-cancelled/). Escape Factory worked on the game between August 2000 and April 2001 thought they referred to it as a demo (Gwertzmann, "What to do when it all goes to hell" Escape Factory Post-Mortem, 2004). The game was reportedly in development for two years so presumably Valve took Escape Factory's work and continued working on it for more than a year longer. The game was ultimately cancelled by Gabe Newell himself, apparently deciding that porting Team Fortress Classic to the Xbox was a better option (McVicker, https://www.youtube.com/watch?v=PuDGHidwlpw).
Although the game was cancelled, a lot of the game's assets were reused for Escape Factory's second game Space Quest which was developed on the Unreal engine. Space Quest itself was ultimately cancelled too.