There are two as yet unexplained enigmas regarding the unusual odour of the urine of those who have recently eaten asparagus.
1) The odour-causing molecules have not been unequivocally identified.
2) Some individuals cannot smell the odour at all.
Reference: Excretion and Perception of a Characteristic Odor in Urine after Asparagus Ingestion: a Psychophysical and Genetic Study in Chem. Senses, Volume 36, Issue 1,