Taking the negative view here, that it might have been held by someone with pee on their hands, Lemon juice works best, better than vinegar at absolutely neutralizing the compounds.
I have cat, who when I am gone too long like to pee on things and the lemon juice, left on to dry eliminates the odor; you may then wipe it with distilled water if you choose.
I have cleaned several mattresses from urine brown to undetectable that way, but I also use a wet-dry vac to remove both later.