Tricky rules question - time sensitive

[Andrew Roberts]

A creature can generally notice the presence of an active invisible creature within 30 feet with a DC 20 Perception check. The observer gains a hunch that "something's there" but can't see it or target it accurately with an attack. It's practically impossible (+20 DC) to pinpoint an invisible creature's location with a Perception check. Even once a character has pinpointed the square that contains an invisible creature, the creature still benefits from total concealment (50% miss chance). There are a number of modifiers that can be applied to this DC if the invisible creature is moving or engaged in a noisy activity.

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