I agree, something like VeraCrypt where the partition has a certain size, with or without hidden data. But state level actors might nevertheless have methods t

I agree, something like VeraCrypt where the partition has a certain size, with or without hidden data. But state level actors might nevertheless have methods to find out, that you write 120gb of data compressed into a 100gb file, there needs to be something hidden because otherwise you would get in 122gb - something like that. Or single stepping VeraCrypt machine code execution (you see I have no clue).