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 (Chem. Senses (2010) doi: 10.1093/chemse/bjq081 )