Pack it in, folks. It was only a matter of time before modders found definitive proof of whether GTA 5's been hiding a jetpack this whole time, and the answer appears to be a depressingly final "nope."
A new GTA 5 PC mod lets you visit all of the otherwise locked interiors in GTA 5, including one-shot mission areas like the Humane Labs and the morgue, and even a handful of areas that never appeared in the final game. As you might have guessed from the last paragraph, not a one of those previously inaccessible areas is a secret cache for the long-awaited jetpack.
James Jarvis and Iain Wilson explain why you can be sure this is the end in the latest GTA V o'clock.
Granted, Rockstar could still patch a jetpack into the game. Maybe one of those substantial story mode additions Rockstar mentioned back in December 2013 will be a San Andreas-style raid on Fort Zancudo's top-secret alien research lab. Or, if you've really bought into the conspiracy theories, it's possible that Rockstar removed the jetpack from the PC version, knowing that modders would discover it nearly instantly and ruin the mystery.
Either way, it seems that you aren't gonna get a jetpack no matter how many sticky bombs you plant on the Mt. Chiliad mural - at least for now.