Spores in the air or contamination through some sort of insect interaction. In an open system, there are billions of microbial agents floating around. With an open container containing a food source and time, the likelihood of contamination is nearly %100.
In biology, molds account for masses of mycelium, that is vegetative filaments or hyphaes and fruiting structures, produced by various fungi like Aspergillus, Penicillium, and Rhizopus. Such fungi, are associated with food spoilage and plant diseases, therefore, when they reproduce, they release spores which are spread to the air and subsequently transported to suitable sources of substrate allowing them to grow again. In such a way, since the jar was uncovered, spores transported by the air were allowed to get into the soup and grow there.
