If you note that some, or any, of your black ABS vent pipes on the roof are capped, there’s a good chance that the contractor forgot to remove the caps after construction. These caps are designed to pressure test the vent and drainage system to make sure they don’t leak as a condition of the building inspection. After the test, the contractor is supposed to remove these caps but this step is sometimes missed. So, if you see caps atop your roof, take them off unless the pipe itself has been abandoned (e.g., from a remodel).