How long does 10mg of methadone stay in urine?

A person that goes to the methadone clinic told me that he took his Sunday dose on Sunday as normal. He took 100mg per day in liquid form(different blend of MD). They drug tested him on Monday and said they did NOT detect any in his system. Accused him of selling it. These people go to the clinic to get dosed everyday except Sunday. They are given a carryout on Saturday and trusted to take it properly. A lot of them will sell it and do without their medicine.