UFO is an event added in the 3.0 SBS update. When this event is activated, it spawns bombs randomly and can spawn up to 3 Dialup Bombs and can drop Soccer Bombs around as well. It used to take on the form as a Mysterious Spacecraft that drops Tacos from the sky. However, in the 3.0 update you can go on top of the UFO as it's spinning around.
After it is done, it will vanish slowly and will not come back.