We started a basement renovation this week and had to demo out an old bathroom to relocate it . The walls of the bathroom were tiled and they had installed Durrock on the walls for the tile.
On the one wall we found this 3″ stack like this. there were also 2 more screws lower in the stack.
At leas with ABS, the screw plugged the hole and did not allow leaks. With PVC, there probably would have been cracks and leaks.