When you are heavily obstructed and someone tries to hide from you, can you find out his location by hearing? What is the range of your passive perception based on hearing?
[RPG] Does perception rely on sight or hearing
dnd-5ehearingskillsvision-and-light
Related Solutions
Passive perception is exactly that, passive. It's what the PCs are always using when not actively searching for something and doesn't use a roll of the die.
To determine if you should use passive perception or allow a player to roll, listen to what they say their PCs are doing. If they say they are standing watch, keeping an eye out or something similar, they are actively searching so they can make a wisdom (perception) check, otherwise they are using their passive perception.
Although, it's kind of an experience call as the DM and depends on what the PCs are actually doing and whether or not the DM gives them the benefit of the doubt. In the case of the video, one guy said he was keeping a look out, thus actively searching and two of them were scouting off to the side of the road so the DM ruled they were also searching, rather than just wandering off into the bushes for no real reason, and allowed those three PCs an active wisdom (perception) check, whilst the others would be using their passive perception.
If someone is hiding, do detection attempts always have disadvantage?
This is a great question. From the rules, the answer appears to be 'No', but it isn't explicitly stated. I'm inferring it from this section of the basic rules for Hiding:
Passive Perception. When you hide, there’s a chance someone will notice you even if they aren’t searching. To determine whether such a creature notices you, the DM compares your Dexterity (Stealth) check with that creature’s passive Wisdom (Perception) score, which equals 10 + the creature’s Wisdom modifier, as well as any other bonuses or penalties. If the creature has advantage, add 5. For disadvantage, subtract 5.
For example, if a 1st-level character (with a proficiency bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Wisdom (Perception) of 14.
Emphasis mine. In this case, the rules are specifically explaining how to calculate the passive perception to detect a hidden creature. Since the rules for hidden dictate that the creature must necessarily be unable to be seen clearly (implying light or heavy obscurement), we can infer that this hidden creature is in some way obscured from the searcher. However, since the passive perception total listed is 14, and does not in any way reference a -5 modifier for an obscured creature, it seems like we can be reasonably sure that detecting hidden creatures happens outside the influence of obscurement. Otherwise, the math present would necessarily have to include a -5 for detecting an obscured creature.
Can you sneak up on someone in dim light according to RAW?
This one is DM dependent according to the rules. We can piece this together from the examples that the basic rules give us:
In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you.
This gives rules for when a creature can detect a hidden creature approaching it. However, since it specifically calls out 'in combat', and since 'out of combat' is not addressed, we have to assume that being out of combat doesn't change the core ruling of the 'hidden' effect (otherwise it would also have a callout, because specific beats general).
The generic hiding rules that necessarily must apply out of combat are:
You can’t hide from a creature that can see you, and if you make noise (such as shouting a warning or knocking over a vase), you give away your position.
and the errata:
The DM decides when circumstances are appropriate for hiding. Also, the question isn’t whether a creature can see you when you’re hiding. The question is whether it can see you clearly.
So in this case, the game is deferring to the DM. You are well within RAW to tell the player that they can't approach the character in dim light from the front and remain hidden. This is even more overt than a simple rule-0, because the published rules specifically call out the DM's ability to overrule it.
My reading of the published rules appear to default to allowing that, but they also give the huge caveat that the DM can overrule it. However, with what we found out above, the detecting creature would NOT receive a -5 penalty by default (unless conditions occurred to warrant disadvantage on the check(s)).
Best Answer
A Perception check can rely on any sense, and your DM decides what is appropriate for the situation.
Perception represents how perceptive a character is in all their senses, not just sight or hearing (from the SRD):
Which sense is used with your passive Perception score or when making a Perception check depends on the sense being used. Even though that will usually be vision (often enough that people tend to assume Perception is always about vision), the situation may call for the use of another sense. You may have advantage, disadvantage, or neither on your passive Perception score or Perception check for a particular sense.
Consider the following mastiff feature to see an example where this differentiation appears in the rules (see Player's Handbook page 307, and note for sake of contrast that many other beasts in the PHB, SRD, and Monster Manual include similar Keen [Sense] features):
This makes sense because dogs have excellent hearing and sense of smell but do not have very good eyesight. Conversely, humans have exceptional eyesight but only average hearing and very poor sense of smell. In the scenario you've given, since heavy obstruction generally completely blocks sight, you may try to use your hearing to locate a creature behind the obstruction, but the DM will have to determine whether the situation is favorable or unfavorable.
Since humanoids are much less adept at hearing than sight, if the DM decides that you can or will make a Perception check based on hearing, they might require you to make the check with disadvantage if they think circumstances are disadvantageous for you to be able to identify the location of the creature by hearing. And if you were to try to identify the location of a creature by smelling their odor or identify the location of a humanoid by sniffing their cologne as it drifts by, the DM would almost definitely rule you to be at a disadvantage.
So, the short answer to your question is: your passive Perception score is as high as the DM rules it to be for a particular scenario using a particular sense. By default, your Passive perception score is 10 + your Perception modifier, but if you have advantage in the case of a particular sense then it is increased by +5, or if you have disadvantage then it is decreased by -5, and your DM adjudicates what is appropriate.
For example, if your Perception modifier is +3, then you generally have passive Perception 13, but you might have have passive Perception 18 if the DM believes that the situation is in your favor for a particular sense or 8 if they believe you are seriously limited in your ability to succeed in perceiving something with that particular sense.
Note that Perception can not allow you to perceive something it is impossible to perceive. If a creature is hidden from view, completely silent, and without odor or other telltale signs such as shadows affecting the area, the DM may rule that it is completely impossible for you to detect them. In that case, a Perception check is not called for because you only roll an ability check when there is a possibility of success, and you would automatically fail on passive Perception as well.