They are very much guided by the heat signature given off by the rodent. If the rat or mouse wasn't warm enough it's likely that she mistook the hand for the food.
As for hissing, it's not uncommon. Many snakes will increase respiration when they are being handled, especially if they get startled. There are also some days that they just don't want to be bothered. This doesn't sound like an aggressive snake to me. I think your best option is to continue as if nothing ever happened, and if you do happen to get bit, it's not the end of the world.
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