What had they stumbled upon?
The burglar suffered from a metabolic disorder known as alcaptonuria. (The scenario is fictional.)
In people without the condition, an enzyme converts a derivative(homogentisic acid) of the amino acids phenylalanine or tyrosine into maleyl acetoacetate, which eventually is broken down into colorless ions.
But in people who suffer from the condition, homogentisic acid accumulates and is excreted in the urine. On standing for a couple of hours, homogentisic acid is eventually oxidized and turns into a dark melanin-like substance. The long-term effects of the disorder include spinal and joint damage.
Archibald Garrod, the physician who also shed light on gout, realized that the condition was not due to bacteria but to some metabolic disorder. Known as black diaper syndrome, it was observed in newborns who lack bacterial colonies. Garrod also noticed that alcaptonuria is rare in the general population (1 in 100 000 to 1 in 250 000) but more common in Dominicans, Slovakians and among children of first cousins. In 1995, Spanish scientists found a gene in fungus that codes for homogentisate dioxygenase. In patients with alcaptonuria a single gene has one of a few possible mutations that lead to the production of the dysfunctional enzyme. It is one of those molecular diseases where two copies of a defective gene must be carried for the disease to manifest itself.
Alcaptonuria diagnosed in a 4-month-old baby girl: a case report. Cases Journal 2008, 1:308
Natural History of Alcaptonuria N Engl J Med 2002; 347:2111-2121 December 26, 2002