I have a sink drain that has four holes inside the pipe. Or perhaps circular depressions would be more accurate. They're about half an inch below the top of the drain. Evenly spaced around the pipe. Each depression is about half an inch in diameter. Two of them seem to be dead ends, about a quarter inch deep. The other two have openings that let you poke around behind the pipe.
See attached photo.
What are these holes for?
See attached photo.
What are these holes for?