Contract: An Elusive Thief - Novigrad
Location: Hierarch Square
Quest Giver: Hierarch Square Notice Board/Sylvestor Amello
Other Requirements: N/A
After receiving this quest from the Hierarch Square Notice Board in the city of Novigrad you will want to head over to the quest marker on your map and speak with Sylvestor Amello (pictured above). He will fill you in on the specifics of this contract. Once you've spoken with Sylvestor Amello you'll want to use your Witcher Senses immediately to discover some paw prints.
Follow the footprints a little bit south of where you are now to the area in between the two tunnels before the bridge. In the corner here using your Witcher Senses you will find claw marks and some food dropped on the ground, inspect them for a quest update.
Continue to follow the footprints to the moat where Geralt deduces that whatever it is jumped down into the moat from here, follow it. The foot prints pick up again on the shore nearby. You'll want to follow them into a nearby house (the door is open) and grab the note on the first floor. What we're pursuing is actually a doppler! On the second floor of the building you'll find some clothes that will give you a strong scent to follow (pictured below).
The Doppler will be disguised as a Guard and he'll be talking to an elf when you find him. Speak with him to make him bolt - chase him down and interact with him to catch him/update your quest. If you let him get too far away you will fail the quest. He'll transform into Geralt and fight you, he's a piece of cake though.
Beating the Doppler will give you a few different options to choose from. You can let him go which will get you some EXP as well as the Diagram: Gnomish Gwyhyr or choose to kill him. Either one of these options will complete the quest for you, although the vendor will be displeased if you can't produce a trophy. Killing the Doppler will get you the Doppler Trophy as well as a Doppler Mutagen, it's probably the best option to take since Dopplers are quite rare.
Tip: Letting the Doppler live gives you credit towards helping monsters in the future quest, Contract: Skellige's Most Wanted.
Return to Sylvestor Amello and talk to him to complete the quest.