one way to help understand ich better is to use the analogy of mange mites on dogs. All dogs carry the mange mite. Whether or not they "break" with mange depends on stress levels, hereditary factors and environmental conditions. (kinda sounds familiar huh?) It could very well be that some things were done just a little differently while you were on vacation and the fish stressed out and "broke" with ich. Not necessarily that anything new was added to the tank, even the sand. Just before, conditions and fish were content and happy and no ich was seen.