Well, as noted, unless you buy into the whole "mass hysteria" thing, which seems exceedingly silly, UFOs kind of have to exist by definition. As to whether the extraterrestrial and/or extradimensional hypotheses are true, well, that's harder. Most scientists don't take them seriously. Me, I'm not willing to rule them out by any means, and I find them extremely fascinating (I don't actually seen UFOs myself), but I still wouldn't talk about them in a scientific context.
As for alien life in general, well, as long as one takes the Copernican principle as a given (there is little scientific reason not to do so), then we essentially have mathematical proof that alien life exists. But do we have any scientific evidence? Well, nothing concrete, but some scattered evidence which is intriguing.