It is tragic to me that this argument conflates "mechanics" with "rolling dice to resolve uncertainty." <snip snip>
My gameplay needs for a secret door are that (a) not everyone notices it, but (b) there's a chance for anyone to notice it, and (c) I want players to have "observant" characters who are better at noticing it. I want to be able to say, "Llyrd the Elven Ranger notices the secret door with their keen eyes."
Core problem: Your foregoing argument and your personal requests conflict, within the context of D&D and its relatives. That is,
in D&D, the only mechanic that inserts uncertainty involves the rolling of dice. One can argue that the intersection of uncertain player intent and uncertain DM response is another "mechanic" that inserts uncertainty, but I don't think that's adequate to meet your gameplay needs here. I'll assume your (a) condition is actually slightly softer than the exact words (that is, "there's a chance for everyone to not notice it"), since as phrased, your requirement is broken if it just so happens that randomness permitted the rare situation that everyone notices it, especially if it just so happens that everyone in the party is an "observant" character (e.g. not just an elven ranger, but also an elven wizard, an elven swordmage, etc.)
So, either we need to create a new mechanic that inserts uncertainty upon request, or we need to accept some kind of not-entirely-uncertain way to add obstacles to success in this context, or we need to accept that "rolling dice to resolve uncertainty" is the only (in-D&D) method to achieve this end.
Dungeon World, the system I run, achieves this with the third option and changes the nature of the situation. That is, if there's legitimately nothing to find, then the player doing the actions that correspond to
Discern Realities will simply be told that their search turns up nothing, without needing to invoke the rules at all. This reflects...not so much a "principle" (which has a formal definition in PbtA games), as a guiding philosophy:
do not roll unless success and failure are both interesting outcomes. If success is impossible/implausible and failure isn't interesting, don't bother with taking up time by rolling, just tell them. If success is possible and plausible, and failure has no meaning/impact, just let the success happen--especially if the success is interesting. Finally, if both success and failure are interesting, include degrees of success.
D&D struggles with degrees-of-success because its roll mechanics are naturally binary. PF2e tries to break out of this by defining critical results (success or failure) as being above/below the target number
by some amount. Knowledge checks in D&D have sometimes used something like this, where there isn't one DC but 3+, and the players get the sum of all the best results they meet or beat. Dungeon World has it naturally built-in.
Finally, failure on knowledge- or perception-related skills is often a Problem for D&D-alikes, because the players know not to trust a result that came from rolling a nat 1. It's not explicitly stated, but my interpretation of how to fix this in DW is that, when the players fail a Discern Realities roll, I
make them ask any one question--and the answer will be one they won't like. Effectively,
something is always learned, but on a failure the thing you learn is a bad fact that you wish wasn't true.
One option, which could theoretically aid with all of this stuff but would be non-trivial to implement, would be to include a deck of cards as an additional/alternative source of randomness. I would actually recommend a tarot deck with some of the cards removed. For example, you could keep just the major arcana, and have each with defined impacts; you could even count whether the card is drawn reversed or not. Invoke the deck when uncertainty is needed but the outcome needs to be more specific/textured than what a die roll can produce.
If we want to eliminate the d20 roll, and still provide players the ability to make their character "more observant than others," what mechanic creates that feeling?
Genuinely curious, because I think a d20 roll to find information is actually pretty kludgy and unsatisfying, but I don't have a great replacement for it, either. I wonder what games based on this kind of mechanic do (detective games, etc.).
D&D-alikes struggle with this, but it isn't just this. How would one provide players the ability to make their character "more educated than others" without involving dice? What about folks who are more discerning, meaning, folks who have better judgment about the correct choice to make? Folks who are more compelling to other people? Etc.
"Passive" checks are one solution, but that's basically just an invisible d20 roll. Another option could be using a character's raw ability score, with some modifiers intended to recognize traits (like "elven" and "ranger") that should reasonably improve that thing. A third is more evinced by 4e, though 5e technically also permits it: trained vs untrained matters, and a trained person can sometimes simply do things an untrained person can't, or can just succeed at something that an untrained person would struggle to do.
Outside of that, there's...really not much D&D offers in this arena, especially 5e, because of the emphasis on "DM Says."
