Here is a scenario for Ability Score: Horror. It is so sketchy it is basically system neutral, and it is broad enough in scope to work in a wide variety of games.
I could not put everything in that I wanted to, but it is an entire investigative scenario on a single piece of paper. Be ready to fill in the blanks. But there are some solid pieces to point in directions and provide easy answers if players go in expected directions. Also, time and date need to be inserted to align with what you are doing in your game; date are like “3 weeks ago” so you can take your current time and fit it in. Same with location of the adventure, and even season.