I won't blame them if they hide the silverware, however. I can be trusted with company secrets, but not with spoons. Never with spoons.
Marc Laidlaw is an author of science fiction, who wrote much of the story of Half-Life, its expansions, and Half-Life 2. Valve's flat corporate structure (at least at the time) makes it difficult to find out who did what. All employees are listed alphabetically in the credits. He hasn't been given a lot of public credit since Half-Life 2, though.
Whatever he's been working on, he has since retired from the company after eighteen years. On his way out, he emailed a Reddit user with opinions regarding his departure, because that's a Valve thing to do I guess. Gamasutra confirmed it's true. It's a relatively short, interview format letter. The Reddit user apparently initiated contact and didn't realize Marc had just retired.
He wouldn't go into too many details about why he left the company, except that he's “old” and he wants to start writing his own narratives. He published several novels before being hired at Valve Software, which he apparently shelved after The 37th Mandala at the short story Catamounts in 1996. He wrote a couple of short stories in the last 2000s, right after Half-Life 2: Episode 2 launched. He wishes to go back to doing that again, which should be a nice retirement pass-time.
What this means for future Half-Life titles? Who knows.
He says that everything's in Valve's hands at the moment, but he could very well have wrapped up involvement in a project just before he left. I mean, it's been five or six years since his last publicly credited work. That's plenty of time to finish an unannounced product. Again, who knows?