There'd be a fair bit of noise if the Holocaust was happening today. It'd be more like Guantanamo Bay or the US's international program of black-bag kidnappings. You don't expect to know exactly what is going on but there is a wiki page [0] and regular leaks [1] even if only a relatively small number of people involved. Something like the holocaust couldn't fly under the radar in the age of the internet; there'd be leaks like no-one's business. They could get away with more radio silence in the 1950s by not having social media.

[0] https://en.wikipedia.org/wiki/Guantanamo_Bay_detention_camp

[1] https://en.wikipedia.org/wiki/Guant%C3%A1namo_Bay_files_leak