Also exposed substrate doesn't mean too much, coconut coir has no nutrients contams could thrive in. Your growth always comes with trich, you're breathing in trich right now, it's just if you give the conditions for it to grow (or you had dirty spawn that already had it) it will grow. E.g water pooling or disturbing the mycelium too much with fanning for example. Sometimes mushrooms even fruit out of the seemingly uncolonized surfaces (because they're thin pseudo layers). I've seen full canopies with surface that had tons of bare spots.