A waved hand says little precisely because the moment contains too much.

At the platform or the gate, language stops working, and the hand goes up because it is the only thing that fits. It is not that there is nothing to say. There is far too much, and none of it compresses into the seconds available, so the gesture stands in for all of it. A small wave at a departure is one of the fullest things a person can do with an apparently empty meaning.

Try not to measure a farewell by what got said inside it. If something was left unspoken, send it the next day, when there is room for the whole sentence.