The pronunciation of "plus" is awkward here. There is a glitch during the "u".
This one is new to me. Is a plus used as frequently as a bientot?
It is more recent in usage (people below 40).
As you have probably guessed, "à plus" is an abbreviation of "à plus tard". The difference is that in "à plus tard" you shouldn't hear the "s" of "plus".
i speak english
you learned it from a book?