If a guard is "on guard", should the "Active Perception" be rolled by the guard, instead of the "Passive Perception"?

Question

Say we have 2 veteran guards, standing inside a room in a mansion, guarding the guild's treasure. These guards are "on duty"/"on guard" 'cause their job is obviously to constantly be alert. The guards have 12 passive perception.

The party wanted to ambush the room! They used Pass Without Trace to sneak into the mansion. Because of the spell, the party rolled 14-22 for the Stealth Check, and 14 is higher than the guard's Passive Perception of 12...

The party's rogue unlocked the door without any noise and the barbarian kicked the door open with a natural 20... So everything was set for a perfect ambush and the party wanted to get the guards by surprise to get the advantage...

But the DM says:

"These guards obviously were "on guard"! That means they were constantly rolling Active Perception instead of just using their Passive Perception of 12. The guards both rolled more than 14 on their Perception Check when you were breaking in, so both of them are not surprised at all!"

I've searched here and found a couple of upvoted answers like this one. Also I checked this pretty popular video from Pack Tactics on this topic. And nobody there even mentioned something like "but check if the guards were on guard and make them roll an individual Active Perception Check in that case..."...

Kindly asking to clarify these questions:

1. If the guard's whole job it to be "on guard", doesn't that mean that DM ALWAYS should use the "active perception check" instead of the passive perception for ALL the "guards" in the world? Just because of their "job description"? :) How about that goblin "guarding" the entrance to the cave?

2. Could you please share a couple of typical situations, when it would be absolutely reasonable for the DM to roll the Active Perception for the guards, instead of using the Passive Perception? For example, if those guards were instructed "there is an intruder in the mansion, stay alert!" or "we heard rumors, that a crazy company of 4 adventures has plans to steal our guild's artifact, expect some intruders today!"...

3. Even if the party was quite aggressive at the beginning of the encounter, and say those guards were directly instructed "There are intruders in the mansion who already killed some of our brothers on the first floor! Be on your guard and protect the treasure!"... But after that, 30 minutes pass and nobody is rushing into the door... Isn't the whole idea of the Surprise that the guards know that the there are intruders in the house, but they don't know exactly WHEN the intruders will rush through the door... So that the guards should "lose their alertness" after 30 minutes and the Passive Perception should be used again?

How would you rule in these situations?

Answer 1

By default, you would use passive perception

The rules about Surprise on p. 189 PHB say:

The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side.

This is also supported by the rules for Passive Checks, on PHB p. 175:

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

So, even when a guard is actively looking around every round, the rules recommend to use the passive Perception if that is done repeatedly over a longer time frame. One such example is for travel, where some characters can use their perception to look out for danger. Noticing Threats on PHB p. 182 says:

Use the passive Wisdom (Perception) scores of the characters to determine whether anyone in the group notices a hidden threat.

So, the general approach is to use a passive check if something is done over an extended period of time.

For what it is worth, it also is very difficult to stay concentrated and alert all the time over several hours. In our own play experience, we use a ruling that you can keep up active perception and get a roll for at most 2 hours. This has worked well for us in practice, and it provides a nice explanation why you have shifts and guard changes, instead of the same guard standing guard the whole night. (But there is no rules supporting this, just our own experience, and related real life practices like having watches change every few hours).

All that said, it of course is in the full right of your DM to rule this otherwise, and decide that he feels it is justified that the guards get an active check, to make this less predictible and more exciting. Especially in situations like the one you describe, where the guards have been alerted to expect intruders that very night, and are guarding a high value target.

The DM can also apply Advantage or Disadvantage to the guards, depending on the circumstances. For passive Perception this is +/- 5.

Answer 2

A passive check represents a d20 roll of 10 plus the relevant modifier, and represents a fair average roll for a creature when they're using an ability for more than a single turn.

It isn't weaker or stronger than a roll, it represents the average roll over time.

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

This feels like your DM wanted something to happen so it did.

Answer 3

My humble thoughts on this:

1. The DM/GM must decide and would decide on the current active state of the guards. Many modules and Home Brew scenarios especially call out that the guards are dozing, playing cards, eating, etc., and not on alert or high alert status. So, in this case, it should be Passive Perception and Rules of Suprise per the D&D Rules.

2. If the DM determines that the guards are on Alert (actively watching) or High Alert (watching, walking around making checks, and signally other guards to ensure the level of heightened awareness then I go with these two use cases:

   2a: Alert = Use Active Perception

   2b: High Alert = Use Active Perception with the DM giving a bonus value on the roll (DM/GM Choice based on the methods the guards are using to actively operate on High Alert)

Answer 4

Maybe No, but probably Yes

If Players were deciding to be actively searching for anything suspicious in a room, would you tell them they only get to use Passive Perception?

They're using their Actions to make Perception Checks. There's no other way to phrase or interpret it, unless you see the game as DM vs Players and are trying to unbalance the game in favor of the Players.

If the Player's would want a situation ruled a certain way for themselves, then it should also function in such a way for NPCs.

However, "on guard" sounds like they might just doing their normal duties, similar to a Player keeping watch. But if they were, say, tipped off that someone was going to break in, they should be rolling, just as Players would be. Passive Perception doesn't mean you cannot choose to make Perception Checks.