They should have sent you a chain of custody form for your drug test along with your offer paperwork. If they are going to do a "post offer" drug test they may give you the info on your first day or after you pass background check. Wait for them to send you the info. Its also possible that not all positions are tested only select ones. My co. only tests management and select hourly positions. Either way it looks like you have bought some time to clean out more for dillution or look in to subbing, Since its a pre employment test it will not be monitored.