In 9.03 I'm No Angel after Castiel becomes aware that angels are hunting him, he gets a tattoo in Enochian which will make him unable to be tracked by angels. This is similar to the Enochian sigils he burned into Sam and Dean's ribs for the same purpose in 5.01 Sympathy for the Devil. However, this tattoo just jams him from being sensed by an angel's normal means of sensing people; Ephraim is able to locate him by sensing his pain, showing the tattoo doesn't block everything.
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