I know the rule is that magic bands can be swapped for FP use as long as their owners have entered the park that day to activate them. But has anyone ever taken a break during the day, then returned with only part of your group and used FP of someone who didn't come back? I assume that Disney doesn't try to see if a band leaves the park and then require it to be reactivated if it returns later that day, but I want to be sure.