In case anyone is still following this post… Today I was working on the patio and noticed there were green stems sprouting out of the remaining pile of straw that I didn’t use. I got to digging around and realized there are seeds in the straw itself (which I assume are from whatever type of plant the straw grows from?). These seeds are nearly identical in size and shape to the popcorn kernels, but they don’t have the same bright yellow color. I didn’t see that when looking through the fruiting bags, but that appears to be what is causing all the growth. Since the straw wasn’t boiled, those seeds survived the pasteurization.
Guess I’ll have to keep that in mind when I do the next crop!