One of my favorite things about the Black Hack is how it treats traps and secret doors, which I think is at least somewhat pertinent here. The BH treats both traps and secret doors as mysteries to be solved. Each has three clues, indexing Location, Trigger, and Action. The BH has the DM roll a d6 the first time a character moves nearby (which is a range band in BH) to a secret door or trap, and on a 1 the character notices something off. In addition, a targeted search based on an attribute test will also reveal a clue.
I tend to view challenge and stakes as two separate but related sliders for determining the need for an ability check. The task itself can be challenging, like the above cliff climbing example, or the stakes and context can be high stress, like trying to pick a simple lock while a zombie gnaws on your leg. Once either slider reaches what I feel is an appropriate difficulty level, I call for a check. I also use those two dials to set DCs. That cliff climb might be mostly easy, say DC 10, but in the middle of an ice storm it might be DC 20. If the PC has climbing gear and the time to use it, I might set the ice storm difficulty lower, or conversely, I might just use the gear to adjust the consequences of failure but not the DC. It's all about what makes sense in the fiction at the time.
The notion of what makes sense in the fiction is also my guiding light for the kinds of investigate checks we're talking about. It can be hard to extrapolate broader concepts from specific examples sometimes. I can talk about how I generally treat the key hidden in the drawer, but the reality is that in the moment, I'll do what makes sense in terms of the fiction. By which I mean based on specific action declarations, prior PC actions and knowledge, and everything that characterizes and contextualizes the narrative to that point. I might very well treat two identical instances of hidden keys in two very different ways. The problem I see with having a rigid idea of what is necessary to achieve X, in this case finding the key, is that it can be very tough to say exactly what a group of PCs will do in a given situation. I'm not going to just give them the key, but I'm also not going to hold it back because someone didn't utter the magic words, if that makes any sense. I prefer to leave room for a broader swath of possible PC actions and ideas to be able to achieve that X.
I tend to view challenge and stakes as two separate but related sliders for determining the need for an ability check. The task itself can be challenging, like the above cliff climbing example, or the stakes and context can be high stress, like trying to pick a simple lock while a zombie gnaws on your leg. Once either slider reaches what I feel is an appropriate difficulty level, I call for a check. I also use those two dials to set DCs. That cliff climb might be mostly easy, say DC 10, but in the middle of an ice storm it might be DC 20. If the PC has climbing gear and the time to use it, I might set the ice storm difficulty lower, or conversely, I might just use the gear to adjust the consequences of failure but not the DC. It's all about what makes sense in the fiction at the time.
The notion of what makes sense in the fiction is also my guiding light for the kinds of investigate checks we're talking about. It can be hard to extrapolate broader concepts from specific examples sometimes. I can talk about how I generally treat the key hidden in the drawer, but the reality is that in the moment, I'll do what makes sense in terms of the fiction. By which I mean based on specific action declarations, prior PC actions and knowledge, and everything that characterizes and contextualizes the narrative to that point. I might very well treat two identical instances of hidden keys in two very different ways. The problem I see with having a rigid idea of what is necessary to achieve X, in this case finding the key, is that it can be very tough to say exactly what a group of PCs will do in a given situation. I'm not going to just give them the key, but I'm also not going to hold it back because someone didn't utter the magic words, if that makes any sense. I prefer to leave room for a broader swath of possible PC actions and ideas to be able to achieve that X.