I can see #1 as a high possibility also, you have a great point Deborah. But still cannot rule out that the substrate as It could be a contributing factor of returning mites as it could be infested with them.
From my understanding you treat your snakes, mites are are deemed gone by your best judgement. You return your snakes back to the substrate right? Mites return if I’m understanding you correctly. As a precaution I would either discard any substrate left in tubs/tanks. And if you have any unused in bags, you can bake in the oven for about 30 minutes around 350 degrees to kill off mites and eggs to insure isolated the possibility substrate is no longer a possibility.
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