Mountains of Misfortune - Meldacio Hunter HQ
First Available: Chapter 7
Other Requirements: Complete Washed Away
Location: Meldacio Hunter HQ
Rewards: 2,000 EXP & Megalixir
This is the second to last quest that you'll receive from Dave; to accept this one you'll need to be on Chapter 7 and have completed Washed Away. Once you've accepted this quest and before you leave town you'll want to speak with the owner of the Diner who will narrow down the quest location on your map for you.
You'll find the dog tag that you're looking for in the corner of ruins which are inside of your quest circle. For the exact location check out my screenshot below, I am standing in front of the quest item.