Highwayman’s Cache is a secondary quest found in the Velen area of The Witcher 3. It has bandits and hidden treasure.
South-east of Crow’s Perch is an island filled with thugs. The bridge to that island is guarded by three bandits who want you to pay a toll.
Following contains quest spoilers. You’ve been warned
Talk to the bandits
The bandits are peaceful, at least at first, so you should talk to them.
Find the Highwayman’s Cache
Use your Witcher senses and follow the footsteps leading under the bridge. You’ll find a diagram and some coins in a chest.