Typically, you would see water that flows out of the basement as clean. It is because they would come from the roof, and the lower part of the building acts as a catchbasin, especially as heavy rainfall. The problem arises when you notice a musty, lingering smell of mildew or mold. The odor is distinctly earthy, damp, and almost swamp-like, and you can most likely smell it from further away as the weather gets warmer, like during the summer or later spring. 

The musty smell is an indicator of a flooded basement, especially when you look around the common area but see no wet spots. It means the clean water became wastewater because it stayed too long in a moist, darkroom. In effect, the mildew forms and consumes waste in its surroundings, such as broken wooden furniture and food waste. You should always call a restoration company right away when you encounter this nasty odor because it can affect your health unless it is dealt with properly.