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Chapter 9: Playing the Game / General Rules / Perception

Detecting Creatures

Source Core Rulebook pg. 465 2.0
There are three conditions that measure the degree to which you can sense a creature: observed, hidden, and undetected. However, the concealed and invisible conditions can partially mask a creature, and the unnoticed condition indicates you have no idea a creature is around. In addition to the descriptions here, you can find these conditions in the Conditions Appendix on pages 618–623.

With the exception of invisible, these conditions are relative to the viewer—it’s possible for a creature to be observed to you but hidden from your ally. When you’re trying to target a creature that’s hard to see or otherwise sense, various drawbacks apply. Most of these rules apply to objects you’re trying to detect as well as creatures.

Typically, the GM tracks how well creatures detect each other, since neither party has perfect information. For example, you might think a creature is in the last place you sensed it, but it was able to Sneak away. Or you might think a creature can’t see you in the dark, but it has darkvision.

You can attempt to avoid detection by using the Stealth skill to Avoid Notice, Hide, or Sneak, or by using Deception to Create a Diversion.

Observed

Source Core Rulebook pg. 466 2.0
In most circumstances, you can sense creatures without difficulty and target them normally. Creatures in this state are observed. Observing requires a precise sense, which for most creatures means sight, but see the Detecting with Other Senses sidebar (page 465) for advice regarding creatures that don’t use sight as their primary sense. If you can’t observe the creature, it’s either hidden, undetected, or unnoticed, and you’ll need to factor in the targeting restrictions. Even if a creature is observed, it might still be concealed.

Hidden

Source Core Rulebook pg. 466 2.0
A creature that’s hidden is only barely perceptible. You know what space a hidden creature occupies, but little else. Perhaps the creature just moved behind cover and successfully used the Hide action. Your target might be in a deep fogbank or behind a waterfall, where you can see some movement but can’t determine an exact location. Maybe you’ve been blinded or the creature is under the effects of invisibility, but you used the Seek basic action to determine its general location based on hearing alone. Regardless of the specifics, you’re flat-footed to a hidden creature.

When targeting a hidden creature, before you roll to determine your effect, you must attempt a DC 11 flat check. If you fail, you don’t affect the creature, though the actions you used are still expended—as well as any spell slots, costs, and other resources. You remain flat-footed to the creature, whether you successfully target it or not.

Undetected

Source Core Rulebook pg. 466 2.0
If a creature is undetected, you don’t know what space it occupies, you’re flat-footed to it, and you can’t easily target it. Using the Seek basic action can help you find an undetected creature, usually making it hidden from you instead of undetected. If a creature is undetected, that doesn’t necessarily mean you’re unaware of its presence—you might suspect an undetected creature is in the room with you, even though you’re unable to find its space. The unnoticed condition covers creatures you’re entirely unaware of.

Targeting an undetected creature is difficult. If you suspect there’s a creature around, you can pick a square and attempt an attack. This works like targeting a hidden creature, but the flat check and attack roll are both rolled in secret by the GM. The GM won’t tell you why you missed—whether it was due to failing the flat check, rolling an insufficient attack roll, or choosing the wrong square. The GM might allow you to try targeting an undetected creature with some spells or other abilities in a similar fashion. Undetected creatures are subject to area effects normally.

For instance, suppose an enemy elf wizard cast invisibility and then Sneaked away. You suspect that with the elf’s Speed of 30 feet, they probably moved 15 feet toward an open door. You move up and attack a space 15 feet from where the elf started and directly on the path to the door. The GM secretly rolls an attack roll and flat check, but they know that you were not quite correct—the elf was actually in the adjacent space! The GM tells you that you missed, so you decide to make your next attack on the adjacent space, just in case. This time, it’s the right space, and the GM’s secret attack roll and flat check both succeed, so you hit!

Unnoticed

Source Core Rulebook pg. 467 2.0
If you have no idea a creature is even present, that creature is unnoticed by you. A creature that is undetected might also be unnoticed. This condition usually matters for abilities that can be used only against targets totally unaware of your presence.