Too much spoilers and no one noticed this… Well, whatever. I'll give you my opinions (like it would matter)...
Spoiler from everything (just in case).
! 1º - It's fiction, just a manga, so the author can cover up this like there weren't anyone normal who noticed, even if it was obvious.
2º - Because Araki (I know it's like the first point, but talking about Araki can't forget writing this just for the lulz).
3º - For Part 5, we can suppose Passione have some connections with the government and asked to cover up this incidents somehow, they are the most powerful and dangerous mafia in Italy, so why not?
! It still is a plothole in some moments, like Straits in New York, but Araki tries to limitate these events only for the characters. Like someone here said, he doesn't want to envolve something more than he wants, so if this happens he just forget and pretends nothing happened, only for the characters he uses.