I play SA since 2004, passed it 8 times, and here are my confirmations:
UFO's: 0.0000001%
Rustler Following you: 50%
Blinking Lights in the sky: 100% (NOT UFOS)
Cave: 98%, there's one in Fort Carson, but some people say it's underground, which is a lie.
Sharks eating CJ: 0.0000001%
Sharks: 20% (I think they're only in VC )
AK Zombie: 0% WTF
Grafitti dissapearing at a certain hour in LS Cemetary: 100% (It actually dissapears at midnight and appears again at 6 A.M.)
Area 69 zombies: 0% WTF
Bigfoot & Piggsy: 0%, no discussion!
Done.