Analyse the assassin's blood to follow her trail | Main story Batman: AC Guide

You begin inside the museum, in the Trophy Room to be precise. Turn on the Evidence Scanner and analyze the big blood stain on the floor #1. Keep holding A until the process finishes. Now you will have to listen to a radio conversation with Oracle #2, regarding the plan to follow one of the members of the League of Assassins in order to reach Ra's al Ghul's body.