Before anyone asks, it is not worth anyone's job not to report it, and later get found out, so the guard/driver would report such occurrences themselves asap.
Are you sure about that?
I work in a depot with over 100 drivers and at least 90% wouldn't report a fail to call or an additional stop unless it was at peak time at a busy station.
Unless you're reported via twitter there's only a slim chance of being found out and if you do then you just play dumb. The punishments don't appear to be any different whether you report yourself or not.