My Theory (Spoilers)
So I think the entire mystery was just a game that Gail, Rachel, and Carlos set up for Doug's benefit.
Doug clearly was bored (random trips to the shore) and lacked motivation (not going to Carlos' DJ gig for no reason, slept a lot, didn't really look for a job, quit studying and cooking). And everyone close to Doug appeared concerned for him. So to get him back to his passion they set up a game where he would have to be a detective to solve a mystery.
Notice how often they play a game in this movie (cards, board game, grapes on the roof, in the car on stakeout). Games are clearly important.
How much of the Mystery does Doug actually solve? Gail mentions the porn site. Carlos makes sure they go back to room 34 and search it and finds the food receipt with the name Jim. When Doug doesn't put together the name Jim and the photo credits, Rachel just tells him Jim is the photographer which makes Doug looks at the pictures. Doug breaks the code, but Carlos is the one who mentioned that Rachel had a baseball Stat book in her room. And how convenient that the code is baseball stats since its clear Doug is into baseball.
When Doug and Gail are following Jim the cowboy they are not very subtle about it. If I had just stolen a case of money I would notice if the car DIRECTLY BEHIND ME made every turn and followed me into a very remote area. Also, look at how long it takes Jim to run out of the restaurant after Gail takes the case. ITS FULL OF MONEY. I would think his reaction time would be a bit quicker than that, after all they are sitting right next to the door. Also notice Gail's reaction when Doug says he slashed the tires. Kinda like uh oh. This all makes me think Jim knew what was going on and playing along.
Also, when Gail is on her date. Notice neither Gail or her date seem overly concerned. And her date starts getting awkward. Almost like he wanted to play along but then got worried he might say something to give the game away so start to back track.
And why wasn't Rachel in her room? That was never addressed. Where did she go? It only makes sense that she was gone because that creates the mystery for Doug to solve.
Anyway, I am not sure if this is accurate at all, but I think it makes the move more interesting to me. I need to watch it a second time. Anyone else have a similar theory when watching it?