Treasure Hunt: Hidden From the Wold is a Side Quest in Velen.

The treasure is located Northeast of the Reardon Manor, near the shore. Eventually you'll come across the deserters camp. There will be three deserters, each of them level 9. Take note that there is an archer and one has a shield.

Once you've defeated the deserters loot the bodies and you will find a letter. Read it then follow the marker to the treasure. The treasure is hiding behind some bushes.