Subtle

A subtle weapon fires either very small ordnance or otherwise generates a nearly imperceptible discharge that even the target may not be aware of. A target hit by a subtle weapon must succeed at a Perception check with a DC equal to 15 + 1-1/2 the weapon’s item level or it doesn’t realize it has been struck. The target notices other effects conveyed by a subtle weapon, such as an injected substance, as normal. For example, you might use a subtle weapon to inject a target with a poison with an onset delay; the target may not realize it has been hit by the dart, but it would notice the effects of the poison once it took effect.