So, before you can start this quest you need to complete Engaging the Empire main-quest, which is the final quest in Chapter 5.
Next, you need to finish all previous side-quests for Dave (see the table of contents on the right), including A Backwoods Burial.
After A Backwoods Burial ends, you can notice that Dave will not leave Old Lestallum outpost. Instead, he will give you another assignment.
Talk to him and you will start the Washed Away side-quest, which involves finding another Dog Tag. The location you have to reach is northern Cleigne.
Since this area is blocked by Imperials, first you will have to unlock it by finishing A Party of Three main quest.
If you already unlocked it, just drive to the location marked on your map.
Objective: Look for the Dog Tag
FFXV Washed Away is a side-quest a bit more difficult than the previous, not because of the enemies you encounter but because of the location where the Dog Tag was hidden.
So, once you reach the designated point, get out of the car, and head to the river that runs along the highway.
Move alongside the river’s shore taking out all Sahagins and Seadevils in the area. These creatures are weak to ice, spears, and normal swords.
The Ice Spear lance, and the Ice Brand sword work very well against them.
Objective: Retrieve the Dog Tag
Once all dangers have been removed from the area, start looking for the Crushed Dog Tag you can see in the image above.
The exact location of this Dog Tag is on the large rocks near river. Look for it behind some bushes.
To locate it even faster, search the eastern side of the blue circle on your minimap, while Noctis is looking away from the river.
The video below will also reveal the location of this Dog Tag.
Objective: Deliver the Dog Tag to Dave
After you find the Dog Tag, all you have to do is to drive back and talk to Dave in Old Lestallum.
Completing this quest will unlock the next one named Mountains of Misfortune; however, since Dave will move to Meldacio Hunter HQ in northern Cleigne, you should first complete a side-quest for Sania.
You can find Sania close to Dave in Old Lestallum, only if you completed A Professor’s Protégé – Yellow Frogs.
Washed Away Quest Info
Description: “Yet another hunter has fallen, and Dave needs help retrieving their dog tag. Noctis agrees to venture into the wilderness and find the tag for the sake of the bereaved family.”
Reward: Phoenix Down x 10
Post Quest Suggestion
Complete A Professor’s Protégé – Gigantoad side-quest.