Tipp: This Contract is a good Target for the Even Odds Achievement
You get this Contract Named An Elusive Thief
(Level 13) in Novigrad from the Notice Board in Hierarch Square. Go to the Marketplace during Day and Talk to Sylvester Amello. Then use the Witcher Senses to the Thiefs House and follow his trail again.
If you let him live you get a good Diagram for a Weapon: Gnomish Gwyhyr Steel Sword (if you have hit Level 42 :D)