It's one of the puzzles. A good way to solve puzzles in Virtual Villagers is to take various villagers exploring aroud the island, especially after tech purchases, and watch for messages about what they see. Keep in mind that villagers with different skills will see different things. A scientist, for example, will notice things that a builder or farmer won't see, while a farmer will see things that the others will miss. Also, the more skilled your villager is, the more they will be able to see.

If you need a hint, or even an outright solution, you can check the Guide.