Funny thing is though, ladders, stairs and various other ways of going from the bottom of a cliff to the top ARE enumerated. Namely under the DC descriptions in the DMG and under various other places in the PHB.
But, the only way to be undetectable that is enumerated is the Hide check. There is no other way that is by RAW, which is what we are discussing, described to becoming undetectable. Wait, I tell a lie. There are things like Etherealness and I believe a few of the monsters might have various abilities.
And, for guidance, we do have numerous examples of things that are not interacting with the environment, make no noise and aren't moving and are set up to be as undetectable as possible - traps. Yet, funnily enough, every trap has a Perception DC. Even magical traps have Perception DC's.
Remember, any DC higher than 20 is meant to be something you see once in a lifetime kind of thing. A PC with a Passive Perception of 25 (for example) has virtually godlike perception abilities. But, it's defeated by a simple 2nd level spell effect? Seriously? This is a guy who can count the dots on a golf ball in flight, but, an invisible target ten feet away totally baffles him?
Yeah, no thanks.
I just did a search in the basic rules for ladders, stairs, elevators and gnomish throwing device DC (kidding about the last two). Nothing. If you're going to quote rules, you should verify that they exist.
I get that you would rule differently that even though a creature or object gives no visual cues, is making no sound, is not interacting with the environment in any detectable way could still be detected. Your choice.
But it's annoying that you insist that I am somehow using my ruling to violate DM Rule #5 "Don't be a Dick" or that the rules say something that they don't. That you have "the one true interpretation" of the rules.
The rules state that you can take an action to hide.
What do the rules say about invisibility?
• When you're invisible, you can always try to hide (from sidebar on hiding in Chapter 7).
• When you're invisible, you're heavily obscured for the purposes of hiding (from the entry for "Invisible" in Appendix A).
• When you're invisible, any creature trying to see you is effectively blinded with regard to seeing you (from the PH errata about "Vision and Light" in Chapter 8 about observers being "effectively blinded" when trying to notice something heavily obscured).
• A creature that is effectively blinded "automatically fails any ability check that requires sight," per the "Blinded" condition in Appendix A.
So we know someone trying to perceive you by sight is going to fail.
What about silenced?
- According to the Silence Spell, a person affected by the spell is deafened.
- According to Deafened: a deafened creature can’t hear and automatically fails any ability check that requires hearing.
So if you can't see or hear, you are automatically going to fail checks based on those two, sight and sound. They are the primary ways to detect someone. No matter what your passive perception is.
The only chance to perceive someone you can't see or hear is then assuming that they interact with the environment in a way that can be detected. Obviously you're going to notice something weird if someone is standing in water. But there has to be something other than sight or sound, which is a DM's call.
This was all spelled out in the podcast that I linked to long ago. That they chose to leave the rules vague and never state that you know where someone is unless they take the hide action.
So while I agree that the primary way to avoid detection in combat is to take the hide action, the rules are also clear that you can't perceive something you can't see or hear based on sight or hearing (and the fact I have to point that out is just dumb). Any other possibility of detection is left up to the DM.