Satoshi would have announced it. I can't imagine he'd want to do something like that un-announced because it would undermine faith in the protocol's security.
https://bitslog.com/2013/09/03/new-mystery-about-satoshi/#comment-1398.. if he wanted to reassure the world that he won’t crash the value of bitcoins, he could simply have provably destroyed the coins (there are several ways to do this and Satoshi would have known them)..