I've checked into non smoking rooms where a prior guest actually smoked in it but house keeping never reported it...had I not reported it immediately upon check in and asked the front desk to come up I've been told I'd have been billed for sure. So maybe the last 1 or 2 guests did and you never noticed it but got stuck with the bill. Who knows house keeping could have given the wrong room number even. Most of the time if you're stuborn and leave an email trail to a Sr Mgr to document your concerns they back off....best of luck and hope you get satisfaction