I Am Very Confused......I Take 10/325 Percocet My Question Is

I am very confused......I take 10/325 Percocet my question is this....... I take 1 tab every eight hours......I have been taking them for several years for severe back pain.......is there anytime that if a urine test is done and you know 100% you take the medication is there any situation where a urine test would come up negative......I am so dumbfounded I take them religiously and my urine test came back negative ?????