WARNING: CONTAINS SPOILERS!!!!!
There is a mural on the wall in the Mt. Chiliad tram station at the top. It depicts a ufo, a cracked egg, and a jetpack. There are also lines and boxes with x's in them. Also, if you go up to the observation deck on the top of the mountain, go to the back side and see scratched in the side of the deck, "Come back when your story is complete".
This started the whole conspiracy discussion. Someone came back after getting 100% complete, at 3am, and during a lightning storm. From the observation deck, you can see a UFO toward the south. If you try to move toward it, it will eventually disappear until you back away. In essence, it is a hologram.
There are, however, two REAL ufo's which are only visible after 100%. The easiest one to see, is the one above the ufo hippies. Take a helicopter from that park and travel straight up. When you get to the "ceiling", you'll see it. If you approach it, your helicopter will lose power until you are a sufficient distance away.
There is also a ufo over Fort Zancudo. I have not been able to see it, as of yet so I'm not sure if the 3am and thunderstorm trigger it as well. This one looks a bit different, more like from the movie Independence Day. Some have reported being able to land on it, but then sliding off.
The UFO above ufo park and Mt. Chiliad have the letters FIB on the side, hence, the whole conspiracy theory that the government is involved.
Now, to the theory. A lot of people think that the mural indicates that there is a jetpack in the game, as yet undiscovered. They have overlayed the mural onto a map of Blaine County/LS and tried to line up the boxes with different areas of the map, placing the top of the mural at Mt. Zancudo or at the sunken underwater ufo which is located offshore north of the map. Also, there are many alien glyphs scattered around the map which people are trying to decode. There are also programmers trying to reverse engineer the code to look for what may hold the secret to the jetpack.
It sure is fun to speculate and look for clues others may have missed. However, IN MY OPINION, this is what the mural represents:
1. There is an easter egg in the game and it is the ufo's.
2. The boxes with x's represent the 5 different areas you need to complete to get 100% - Missions, Hobbies & Pastimes, Strangers & Freaks, Random Events, and Miscellaneous.
3. Once you complete all of the areas, it will unlock the ufo easter eggs.
4. The jetpack represents flight, not an actual jetpack. There is a line connecting the jetpack image with the ufo image, meaning, you need to FLY to see the UFO's.
The believers cannot be swayed. The non-believers cannot be changed. But it is a fun diversion if you're interested in pursuing it.