I have an unfinished bathroom that has a sink and toilet on one side and a square area with gray rock filling it and a 2.5 inch PVC pipe that sticks up. When you dig to the bottom of the gray rock it has a angle tube and then goes straight into the concrete floor (I don't see any type of U trap). We have noticed for the last year when we live here an occasional weird smell from the basement and I feel like it comes from this room. It may be associated with a lot of washing/water use. We are on a septic tank. I have had this tube covered up with duct tape but am curious on what you think this tube is for (was thinking maybe possible shower in the future) and if I should be covering it with a PVC cap or if that would cause any issues with covering up with a cap

if taping the end of the pipe stops the smell, just get a rubber test cap that tightens with a hose clamp..

Would there be any issues capping it off, ie is there some type of PVC pipe in a basement that would need to vent