In this theory we have to check the Concrete Jungle, Cold War, Dark River comics/books and the Predator: South China Sea novel and the Predator 2 film.
So, let's start what we know!
After the incident in Val Verde the Other World Lifeform Taskforce capteures Alan "Dutch" Schaefer. However in the movie, Predator 2 (set in 1997) Peter Keyes mentions that Schaefer escaped and they don't know where is his current location. And this is what we are know, fortunately the comics/books help us solve the missing Schaefer problem.
The Concrete Jungle, Cold War and Dark River arc base around Schaefer's brother, Schaefer (yup, there are two Arnold Schwarzenegger like guy, BUT! the Concrete Jungle book mentions that Arnold is an in universe actor, so there are three - 3! - Schwarzenegger out there). Detective Schaefer's goal to find his missing bro, and he came really close to this in the end of Dark River. The comics takes place BEFORE Predator 2, so piece together this: we got Alan, who somehow escaped from OWLF captivity before/during 1997 and we got detective Schaefer who knows his brother's locations and wants to free him.
But what happened after the escape? Check out the Predator: South China Sea novel. In it, we see a misterius person, nicknamed Onyx. He is working under the radar, John Gustat (one of the main chatecters) hardly could find him and he never sees Onyx's face. However this so called Onyx knows a ton of information about the Predators, he even mentions that the mud saved his life against a Predator (aka Dutch from the film). Also, Onyx love smoking, just like Dutch.
So in conclusion: det. Schaefer freed Alan, who later becomes a shadowy figurine, who helps others to kill Predators (if they pay him few million $, but in exchange Dutch give out special equipments)