When on vacation, a Sim can rent a Metal Detector from Rental Shack Zack and go looking for buried treasure. Once the Sim stops looking for treasure, the metal detector returns to the shack and must be rented again if if the player wants that Sim to keep digging. More often than not, the Sim will find small amounts of money that was presumably lost by other vacationers. However, Sims may find souvenirs while looking for treasure. If they're lucky, they may even find a bit of treasure. Of course, they may also find an old boot or two.
With Unleashed, a bored dog may dig holes. Since vacationing families take their pets with them, there's a slight chance that a dog will dig something up while on Vacation Island, but it's not very likely.
The Sims 2
Digging was added in The Sims 2: Bon Voyage. Digging can yield unexpected treasures, maps to secret lots,rocks, bone, or bust the water main etc.[TS2:BV] Sims can dig on any terrain anywhere for example dorm, hotel, home, or any community lot. There is the random chance that your Sim while digging can bust the water main if this happens the sim will run around and panic while the busted water main will create puddles and a huge mess and lower the environment score, the only way to stop the water from spraying is if the sim fills the hole in.
Stray dogs or your sim's dog can also dig holes in the yard with the addition of The Sims 2: Pets. They will lower the environment score. Sims or servants can fill in these holes.