Handling encounters where the monsters aren't initially visible

Question

When researching how to handle invisibility, I came across the Rules of the Hidden Club which state: "The First Premise: Everyone knows where everyone else is, at all times, period.". But I can't find that rule in the official rulebooks. How do I handle encounters where the monsters aren't initially visible?

Basically I am wondering whether I handled the following situation correctly: The players are moving into a room in which they had previously (days before) encountered monsters, but ran away. What I did was let the players move their characters in the first round as if in combat, but not show the positions of the monsters, which were out of sight (some inside sarcophagi, others behind walls). I only put the monsters on the battlemap once they started moving or coming out of their sarcophagi.

Where in the rulebooks does it say that the players always know the positions of all monsters on the battlemap? Should I have put all the monster miniatures/tokens on the map as soon as the players step on to the map? Or would it be okay to let players enter a room "out of combat" (using what movement rules), and only show the monster positions once the monsters reveal themselves?

Reading the question and answer BESW linked to, I found what is probably the relevant rulebook section on page 223 of the PHB2: "Invisible Creatures and Stealth: If an invisible creature is hidden from you (“Stealth,” page 188), you can neither hear nor see it, and you have to guess what space it occupies. If an invisible creature is not hidden from you, you can hear it or sense some other sign of its presence and therefore know what space it occupies, although you still can’t see it."

However it feels strange to apply this rule out of combat. The players enter a tavern and immediately know the location of everybody in the tavern, even those upstairs in a room with closed doors? I don't think so! Shouldn't it be possible for the players to be on a battlemap of a room they explore "out of combat", with the encounter starting as soon as the first monster jumps out of hiding?

Answer 1

4e is not a simulationist system. Its combat rules are specifically that, rules for combat.

The majority of 4e's ruleset (including all the various rules dealing with stealth and detection) is only concerned with what happens either right before or in the ensuing combat after an initiative roll. There are ways in system to handle the situation you were trying to create.

In the tavern example you give, yes if a combat were to suddenly breakout they would detect everyone in the same room as them, but probably not people upstairs behind a door, whispering of nefarious deeds. Unless PCs are making active checks this is against their passive. Thus in the situation above It would be the Hard DC (For their level) +10 to detect those people whispering in the room upstairs, probably would not happen.

Published in Player's Handbook, page(s) 186, Player's Handbook 2, page(s) 223, Rules Compendium, page(s) 150.

You should re-read the sections on perception and I think you'll find that it generally makes sense, everything is against passive until a character declares they are actively checking.

A more effective way to run your encounter or a similar one next time

Each creature you wished to be hidden from the players would make a stealth check in their hiding places (superior cover). You roll these all secretly and note them. When the PCs enter the room you compare those stealth check roles to their passive perception. You should have copies of their character sheets or already have this noted. If you have to ask them for their passive perception you've tipped your hand and even the best roleplayer will be tempted to start rolling active perception checks.

If any of the PC's passives beat the stealth checks, pass them a note saying which squares/sarcophagi seem to have something inside. You should probably then roll initiative at this point. Any undetected creatures remain hidden. So long as the whole of the party did not detect at least 1 creature, a surprise round occurs. I would have all of the monsters that were trying to hide (both those successful and those detected) spend their surprise round action to ready an action to attack the first player that gets within their charge range or something like that. The surprise round initiative goes through and then the regular initiative begins. They may not get to use their readied attacks, but they can still be hidden and/or invisible but detected.