I found this from the KA website.
Catching a "Wild" Yeast of Your Own
A second way to get a starter going at home is to capture the wild yeast that resides in your own kitchen just as the Mediterranean baker did. Capturing wild yeast is fun though a bit unpredictable. The summer and fall are times of the year when there will be more of them around. If you bake with yeast fairly often there may be enough wild yeast in your kitchen to activate a starter. If you can afford the time and the haphazardness of the results, it's worth a try. When you've captured some wild yeast successfully, you'll feel very accomplished. Here's how to set your trap.
• 2 cups warm water
• 1 tablespoon sugar or honey (optional)
• 2 cups King Arthur Unbleached All-Purpose Flour
Mix the water, flour and optional sweetener together thoroughly in a clean, scalded glass or ceramic bowl. The scalding will ensure that you're starting "pure." Cover the bowl with a clean dishcloth. Put it in an area where there's apt to be the highest concentration of airborne yeast as well as the warmth that is needed to begin fermentation.
If the surface begins to look dry after a while, give the mixture a stir. It should begin to "work" in the first day or two if it's going to at all. If it does, your trap has been successful. As you would with a dried starter or active dry yeast, let this mixture continue working for 3 or 4 days giving it a stir every day or so. When it's developed a yeasty, sour aroma, put it in a clean jar with a lid and refrigerate it until you're ready to use it.
If the mixture begins to mold or develop a peculiar color or odor instead of a "clean, sour aroma," give a sigh, throw it out and, if you're patient, start again. Along with the vital yeasts, you may have inadvertently nurtured a strain of bacteria that will not be wonderful in food. This doesn't happen very often though, so don't let the possibility dissuade you from this adventure.
The link to the full page can be found HERE http://www.kingarthurflour.com/tips/tip.php?id=48481
You are correct in that the hazard is probably minimal because it's a cooked product. However, there are bacteria for example that live in underwater lava vents. The point here is that people assume that because they are using a specific food source to attract them, they will attract ONLY critters that like to eat on it. This is probably a fair assumption, however in the Biology world there is a mantra that states "what CAN happen, eventually will." Part of the essence of bacteria is that they fill virtually every single niche on Earth. They mutate rapidly and take over niches. Part of the hazard here is also not the food per se, but keeping that stuff in your fridge, or in your house with other food that you eat raw, such as veggies and fruits.
I am not saying the risk is high, it's actually probably quite low, but just know what you are growing. Is it a mold or a yeast? That sort of thing. I mean no one wants to be that .000001% person that died of some mutant pizza, or inhaled mold spores that gave them a respiratory infection.