It's possible that they've already reflown returned engines without telling anybody. There are some indications that this has happened, and it would be a fairly

It's possible that they've already reflown returned engines without telling anybody. There are some indications that this has happened, and it would be a fairly safe thing to do on some flights (since the Falcon 9 can deal with an engine out situation, even at T+0). It's likely they'll do just that with engines and other components in the future as well, such as landing legs and grid fins.