Buenas?

Buenas?
 

I am currently trying "passive listening" to Spanish. After "¡Hola a todos!" someone says "Buenas". In German someone could (at least there was a time when it happened) hear "Guten" in the German state "Hessen" they still "Gudden" - shortend for "Good Day" is it the same here? Instead of the full phrase "Good Day" / Buenos Dias you shorten to "Buenas"?

Yes. Exactly it.

Be aware though, that most of the time you can say that, either, early morning or at night. Why? because in the morning, for example, you can hear some people say "buena mañana tenga usted", not too common but some people could say that, and definitely you can hear that at night "buenas (noches)" only when greeting, not bidding goodbye.

In Mexico it can only be used for afternoons and evenings (it substitutes "buenas tardes" and "buenas noches", but never "buenos días"), and as Chileno points out, only for greeting.
However, some people here consider it low register and should be used only in obviously informal contexts. Saying "hola" can be much better for most social environments though.

Sabía que se me estaba pasando una opción... buenas (tardes).

Ocupado como siempre y apurón para contestar.

Que se le va a hacer.

"Buenas" occur in a Spanish session between a teacher and his students. Most common were "Hola" or something similar but "Buenas" occured at least one time and made curious.

Thanks for explaining and even for the step further saying when it's allowed etc.