Catch Someone Napping Meaning

(idiomatic) To take advantage of someone's inattention.

Example: 2011 January 25, Les Roopanarine, “Wigan 1 - 2 Aston Villa”[1], BBC:
  Wigan keeper Ali Al Habsi was forced to deny Stewart Downing and Bent in quick succession, and again got down well to deny Bent after a Young free-kick had caught Steven Caldwell napping.