A "surprise release" refers to when something, like a new song or movie, is unexpectedly launched or made available to the public without any prior announcement or promotion. Full definition
While they did not reveal any new games in the series, they have offered plenty of updates on previously announced projects, as well as surprise releases of older titles. (gamesnosh.com)
If this is the level of what we can expect then maybe the next surprise release should be not releasing it at all. (screenrant.com)
E3 has provided us with two surprise releases however, both of which are available right now. (gamesasylum.com)