)rStrangelove]
a.) they have missed us (not unlikely since we're in a remote sidearm of the galaxy)
b.) they are hiding themselves (maybe there's 1 very advanced species out there exterminating everything else, like galactic grasshoppers)
c.) they have seen us but chose to not show up in public yet (very likely)
Based on the evidence we have*, the following statements have just as much merit as the above three.
d.) We are the only life in the universe
e.) Life is so incredibly rare that the communication distance makes it seem like we are the only life
f.) Life is incredibly abundant, but very very few planets develop intelligent life.
g.) Intelligent life is also incredibly abundant, but we all developed the same technology around the same time so none of there signals have reached us.
h.) There were intelligent societies that had very abundant radio waves to listen to... in 100000 B.C.E. and they have all died since then.
Now, I think the probability of intelligent life in the universe is 1, as humanity is, in fact (at least to our standards) intelligent. Do I believe in intelligent life besides Earth? Well, there could be, but we don't have sufficient technology to make a meaningful conclusion.
Do I believe aliens have visited Earth?
No, we have found no remains of alien technology, remains, or anything that would indicate an alien presence here. And unlike finding aliens out there, there's very little room for where there could be alien stuff hiding here so the most reasonable conclusion is that we are not.
What would I ask an alien should I meet one?
1. How did you find us?
2. How long have you been here so that we can hold a conversation so easily?
3. How do you speak our language easily enough that I can understand you?
4. Does P = NP?
5. Do you have a proof for that?
Though it would be more likely that our first meeting would go like this:
Alien: OMG it has intelligence!
Me: Ba wha ah uh hum *faints*
* i.e. none at all.