DIY Doctor UK DIY Forums

I am getting a distinctive smell in a first floor bathroom served by an internal soil stack if I close the bathroom window and door for a few hours. I would describe the smell as antiseptic or cleaning fluid like. The soil stack is vented externally above the house roof eaves. The drain pipe the soil stack is connected too also takes waste water from a washing machine on the ground floor. Could the smell be from a small leak in the soil stack. If so I would have expected it to smell bad rather than what it does? I have checked the waste water traps, there are no water leaks visible, the soil stack appears in good condition and drains inspection chamber shows waste water is flowing freely. If it is not from the soil stack I am at a loss from where the smell is originating from and other bathrooms in the house do not have the problem. The bathroom is question has a toilet, basin and shower connected to the soil stack.