If you look back at what we've previously been taught - or look at the drop downs, See you = Tot Ziens. So why don't I need to say 'Tot ZIENS vanavond'? Is the ziens just assumed, in a form of shorthand?
The English phrase "see you" (short for "I will see you") is translated to the Dutch farewell "tot ziens" or, if it refers to a specific time, "tot <time>". Literally, this just means "until this evening".