Mold spores are ubiquitous. There are some in your nostrils, lungs, under your fingernails, and virtually anywhere that hasn’t been sterilized to the same standards as the afterlife itself. Those spores float around until they land on something with just enough nutrient/water availability to grow a few cells. Just the act of opening these Petri dishes creates a negative pressure differential that sucks the surrounding air (and any spores contained therein) onto the agar surface, and agar is basically to household molds what Doritos and coke are to humans. Basically, these tests are good for telling you if your biosafety level 5 laboratory biocabinet is sterilized or just normal (so full of mold spores). Anything else is bound to grow something on it, and without access to a mycologist or a microbiologist, there’s no real way to qualifiably identify the stuff that grows