I'm not convinced that these people are organized and competent enough to kill people in that scale without people finding out.
That said, I am certain that a whole lot if people have been killed in custody and covered up, but I don't think it's on a scale like that.
The Germans abandoned the Minsk ghetto before the end of the war and left about five thousand piled up charred bodies in the middle of town because it wasn't a real death camp and they didn't have cremation ovens, so that's what you're grandmother apparently didn't notice.
https://encyclopedia.ushmm.org/content/en/article/minsk