So, this might be a bit of a dumb question but sometimes I text someone and then I get the ellipsis back and then I usually get a message. However, sometimes I never get a message back and am curious. Does this mean they started typing something they didn't send? Is it possible that they are texting someone else? More curious than anything.
Most likely they started and stopped. Maybe they saw you typing at the same time and you both decided to wait for the message before responding? Theoretically it should only show when someone is typing a message to you.