There is a two volume sefer (very small and thin) called מעתיקי שמועה, where they are מלקט various Chiddushim of the Brisker Rav from his Talmidim – from Shailos UTeshuvos, from Sefarim al haShas and other sources. I saw it there.
The sefer is out of print, so it is quite hard to get hold of. It is very possible I’ll have access over Shabbos, so I will try look for the original source.