Use a submarine to reach the north part of the map in the Pacific Ocean in. Around there you can find an crashed UFO deep underwater.
In a helicopter, fly at a very high altitude over Fort Zancudo. Jump out and skydive to possibly have a UFO materialize below you. Note: Attempting touch the UFO will result in you getting repelled by a force field or killed by the propulsion system.
After getting 100% game completion, go to the viewing platform at the top of Mt. Chiliad. Wait until 3:00 a.m. on a rainy night (cycle through the "Change weather " code if needed), and a UFO with FIB markings will appear in the sky. Note: The mysterious hologram will also appear in the distance, but the UFO will disappear if you approach it.
Fly a helicopter at a very high altitude over the hippy camp at Sandy Shores to have an FIB marked UFO materialize.