Just becuase you didnt see them does mean they weren't there. You instinct to treat everyone of you snakes is a good one and you should do that. If your coastal was in the same room as the other snakes then they would most likely have them too. They might just not be noticeable in the other tanks yet. It you just treat the coastal most likey it will be another few weeks and then you would see another explosion in one of you other snakes.