few question the existence of aliens... even if they are just single celled. We question whether they are among us. It's 4.3light years to the next closest star. Let's face it, even if we sent out signals in every possible direction we likely wouldn't year back for decades or even centuries and even then, they are probably just as bound to their planet as we are to ours.
I would totally agree, except that few question the existence of alien life. I probably would have agreed with that last week though.
I wouldn't make a determination that intelligent alien life exists, although I do suspect it's likely. It's a totally different argument though, focusing on the evolution of animals and the evolution of intelligence. However, since alien life in general probably exists, intelligent alien life is as likely an assumption as to assume that since we have rockets and have put a man in space, we could probably go to the moon as well.
UFOs, which are closely linked by many to intelligent aliens, are more likely humans using very advanced classified technology and/or doing classified scientific experiments. I don't believe it's a coincidence that the UFO craze started in the mid-late 1940s and throughout the 1950s, when advancements in aerospace and propulsion systems took gigantic leaps. Still, I couldn't rule it out, or say that it would be all that surprising if they had visited Earth. Given a few more thousand years, or even a few hundred, it's mind boggling what humans might be capable of. However, there's not enough evidence to make a good bet for, or against.