When your running around doing quest or exploring your dog might bark, this could be because or Treasure, Quests, or a Dig Spot. You can tell which it is because of the word and picture above your dog's head! If there's a dig spot follow your dog over to it and when he starts digging hit the A button. Now do this 49 more times and Bleep Bloop Achievement Unlocked :D
Note: Dig Spots can only be dug up once!
Note: Dog training books can also be found to improve how often your dog find's items
unSATISFYing commented that there are also relationship quest's that can involve digging up a hidden item, to get these quest's do a few nice expressions on a person and they should give you a relationship quest. Now it might not always be a digging one though!
Salvatore 2 commented that you should go by the little box that pops up on your screen sometimes saying how close you are to the achievement NOT the stats book in the sanctuary