Anything unannounced is a total surprise. If your neighbor frequently barges into your house unannounced, especially in the middle of dinner, you might want to start locking your doors.
When an organization doesn't make a formal statement before doing something, it's often described as unannounced, like the unannounced change in the ingredients of your favorite soda or the unannounced delay of your flight to Hawaii. You can talk about anything unexpected using this adjective: "Their unannounced marriage ceremony took everyone at the picnic by surprise."