Strangely, I was thinking. (This fact itself is THE new element I'm giving to the forum!)
Maybe the mural is showing three outcomes, three prizes, for the three possible endings the game allows you to choose among: a Jetpack if you kill Trevor/Michael (connected to Trevor, the specialist in flying), an UFO if you kill Michael/Trevor (connected to Michael, the specialist in dealing with aliens and wannabe-stars such as cinema workers and kifflom-speakers), and a bunch of meaningless easter eggs to look around for you if you don't kill anyone (or will it be an invasion when all those eggs/shells open and aliens or dinosaurs emerge from them?).
If this interpretation of the mural's meaning has some truth to it, it might well be that every player looking around for clues could possibly have three different results depending on what final s/he had chosen for the final mission, either just as a final "prize" or for each clue and related procedure which should lead to the final prize (I mean: maybe each prize has its own path, its own series of clues and conditions to fulfill).
I really think you are right, about the Xs, there are five missions where you can chose different ways, the jewel store, fbi, merrywater, bank in paleto bay and the other big bank. Maybe this missions trigger different easter eggs
If any of these conditioning events have an impact on clues and easter eggs activation/working, it will be a lot more difficult to crack the mistery, as players should split into groups with the same story-mode choices in order to expect the same reactions from the game when they are looking for clues or trying any procedure which is aimed at finding the solution to the mistery.
That's the point of my post and the reason I wrote it. But I'm positive that some other member posted something similar about similar interpretations to the mural meaning (I couldn't read the whole thread, so I'm not sure about it...).
- Sorry for the errors: English is not my language