OK, had to put the inflated tube completely underwater and squeeze it to increase the pressure as much as possible, but I finally found the tiniest of holes. It's completely invisible, and even with being underwater and squeezing it, only leaks the tiniest bit. Now I can at least patch it, but why does this keep happening repeatedly and can I prevent it? It must be a form of a pinch flat even though I only found 1 hole and not 2 like you usually see. I guess fatbikes must be prone to them because of the low pressure ... an argument not to use tubes, I guess. I always thought tubeless was primarily to save weight.