So... This is the first time I've heard "...plus tard" witha silent 's'. It makes more sense to me than what I've been hearing so far, but it's got me wondering -- which is more widely used?
I've also heard the 's' in 'plus' pronounced in other contexts (same with the 's' in 'tous'!)... What's going on? wide-eyed pout. whimper
I can never resist a wide-eyed pout combined with a whimper. I was going to just share my limited knowledge, but googled instead. This is quite a full answer: http://www.fluentfrenchnow.com/how-to-pronounce-plus-in-spoken-french/ I'm glad I didn't just guess, because, as it says there, "La situation est plus compliquée que je croyais."
It isn't "the girls" because that isn't good English. In the sentence, "girls" is addressing (talking to) some girls. So just as you would say "Goodbye, Sally and Samanthat!" you would say "See you later, girls!". Similarly, you would say "Girls, come here!" or "Mind the car, girls!". It sounds strange to native English speakers that the French includes the definite article, so I imagine it sounds strange to French speakers that we drop it sometimes. We would never address someone as "the girls", only refer to them that way. English often drops the article where French requires it.