Why would morphine not show up on a drug test?

Methadone has a half life span. That means that whatever dose you took, that the next day half of it is only left in your system and then each day after that your body eliminates half of the dose until it's cleared and it also depends on your metabolism, what you eat, drink and the amount of methadone taken and for how long. When you did the urine test, they may not have checked for it or your body got rid of it in time.