The phrase “hidden treasure” is too often used to describe places that really aren’t, but in the case of Pescatore Seafood, it’s absolutely accurate. For one thing, Pescatore is hidden: It’s at the less-traveled edge of Somerville’s Ball Square, tucked behind Kelly’s Diner in a pie-slice-shaped building at the corner of Rogers and Boston avenues. But even once you find the building, the clean but uninspiring takeout counter at the front gives no clue that there’s a 24-seat dining room around the corner. The dining room is a lovely, calm area with sandstone-colored walls, a few sepia-toned photographs of Italian street scenes on the walls, and one very large vase on the floor. It’s not fancy, but it’s very pleasing, with a touch of sophisticated restraint – a lot like the food that’s served there.