Let me just clarify, I don't think the purpose of the jetpack and UFO boxes are to say "Hey, these will be DLC soon!". I think they are those symbols, SOLELY because they represent the location they are showing. Rockstar used a Jetpack symbol because they knew what effect it would have had on the people.
With all the themes in the game, I seriously see a Military/Government vs Alien Invasion DLC pack. Whether or not the Jetpack is given to us there will remain to be seen.
Regarding your statement, when you said the UFO was directly above the fake UFO on top of car, this isn't true, I just checked to see what the UFO was directly flying over and it turned out to be this circle with 3 stars on it.
Three stars, each star in the same (or nearly the same) colors as the games three stars, all in or around a circle.