As others have said, it's for a bunker supply mission. Why you saw it when you were alone, I don't know, it's a glitchy game.
Come on, you can't call this game glitchy. Play early access game for comparison, or other AAA games. GTA for its scale doesn't have that many bugs.
Lol, just lol
I agree with Maned on this one, Hell Ubisoft games have less bugs then GTA Online, that's still at Public Beta levels of Bugginess, That's saying a lot, cause at least those other AAA Games make some sort of try at fixing their goddamned game, R* Just said f*ck it, and only patches what affects the flow of Shark Card Money.