I am putting a bathroom in my basement, which is already stubbed in. When I cut the cap off the 4" toilet drain, I notice a clear 1/2" piece of tube in the middle. I thought it might be some scrap dropped in during construction, so I reached in to pull it out. It wouldn't budge. After further inspection, this 1/2" tube runs the length of the drain pipe. I can blow into the tube and air passes through, so it is not blocked off at the other end. I have been remodeling for 15+ years and have never seen anything like this. Any ideas on what this is for?

That makes 2 of us, never seen anything like this. I would have bet my right arm your first thought was correct, scrap. You can't use this until your find out what is holding this tubing. It may require you get a SeeSnake camera view of the drain. Ridgid 25643 SeeSnake Micro Inspection Camera Dwos REPLACED by 632-311 I bought one of these at Home Depot and have found many uses, you can rent them too.