The Oruch HaShulchan O. Ch. 132 (1) points out that Chazal did not institute to say “kedusha” at Maariv. Indeed we do not say “kedusha” at Maariv; but on Motzoei Shabbos we do say ואתה קדוש וכ׳ in what is called the “Kedusha Desidra”. (It is also said any time there is a post-Shmoneh-Esrei Nakh study (Tehillim 91, Esther, and Eikha are the three currently practiced examples for Maariv; at other prayers it's Tehillim 145/20)).
Normally in the “Kedusha Desidra”, the verses of the kedusha in Hebrew (e.g. קדוש קדוש קדוש ה׳ צבא.ות וכ׳) are said aloud.
I am informed that there is a (yekkische) minhag not to say them aloud on Motzoei Shabbos. Is the fact that normally Chazal did not institute to say “kedusha” at Maariv the source of this minhag?