As part of a full bathroom renovation, I am removing a mid-section half-wall that separates toilet/shower from vanities/cabinets (now removed!). In the process, I discovered a white PVC pipe with a label "Not For Pressure". By a quick inspection, I see no exhaust on the roof around the same area, nor I see any water heater exhaust coming from the level below to this bathroom. I don't think this pipe carries water. So can someone chime in with any insights or ideas? Thank you!