In Liminal, if someone is searching a room for a clue I want them to find, I'll have them roll Awareness. If they succeed, all good. If they don't, then while simple failure is an option in Liminal (and the one that's used for a lot of rolls), if I really want them to find the thing there are options.
"As you pick up the book you need, a pile of other books falls to the floor with a series of loud thuds. You hear voices and running footsteps from upstairs."
"As you reach into the hiding place your hand closes around something sharp. You retrieve both the caltrop and the now rather bloodstained document you were looking for."
"After a long and exhausting search, you finally locate the hidden drawer. When you look up, you realise that the sun has set."
I think what I find fun about writing and running an investigation is the structure. My current convention scenario, Mother Said I Never Should, is based around investigating the disappearance of a teenage girl. My scenario notes consist of two things:
- The sequence of events that lead up to the current situation.
- A set of clues and the different ways they can be discovered.
My previous convention staple, On The Drift (a Firefly adventure for Scum and Villainy) isn't specifically an investigation - it's about the PCs getting control of an abandoned ship. However, it can become an investigation, depending on how interested the PCs are in how exactly the ship came to be in its current state. I have a pretty similar set of notes, in that I have the sequence of events that led to the situation, plus a set of clues. They're a bit easier to find than in the Liminal scenario - mostly just a case of looking in the right room - so rather than the different ways a clue can be discovered I just have a note of what's in each room of the spaceship.
Since this isn't an investigation-focused game, it's entirely possible for the players to decide they don't care what happened and go and do something else (and it's happened, and I'm prepared for that too.) But it's a lot of fun for a group who do want to play it that way.
The nice thing about writing an RPG investigation is that unlike writing fiction where you need subtle clues that take a brilliant detective to figure out but a reader can identify on a second reading, you can go for much clearer clues. And with a game with supernatural elements, you don't have to worry about why the police missed all this stuff, because the police are operating under a different set of assumptions to the PCs (e.g. the culprit is human size, solid, visible and subject to the laws of physics.)
Conclusion: I should write more investigation scenarios. I know how to structure them, I enjoy running them, and based on the feedback I'm getting, people enjoy playing them. And I'm part way there already, with my 'in progress' folder containing a partly written Alien scenario with the working title CSI: Weyland Yutani.