There actually isn't a requirement that pardons be announced until they get someone out of the consequences. Trump could have pardoned himself for any federal crime, and just left the document in a drawer somewhere to be used later.
The question of constitutionality has never been tested, so maybe it works and maybe it doesn't. As others have noted, it wouldn't work at all for state crimes. But as a concept, "secret" pardons are conceivably possible and potentially valid. We just never had a criminal so brazen in the White House that anyone thought of it before.