I've been involved in the study of the UFO phenomenon at a very deep level for almost six decades. Some 48 years ago - after I'd been born again - I experienced a close-up UFO (a daylight disk at a range of about 50 yards, not some "unusual light" in the night sky) in the company of a guy who thought my religious beliefs and UFOs were both nonsense (until he saw the UFO). People I know and respect who have no interest in UFOs have reported experiences to me that were absolutely mind-blowing.
Despite my studies and experience, the UFO phenomenon has had essentially no effect on my religious beliefs. The speculation about UFOs possibly being demonic dates back at least to Rev. Barry Downing's 1968 book The Bible and Flying Saucers. Such speculation continues today. Nick Redfern, who is a pretty reputable thinker in this field, published a book in 2011 suggesting that a highly-placed "inner circle" with the U.S. government believes UFOs are demonic in origin (FINAL EVENTS and the Secret Government Group on Demonic UFOs and the Afterlife).
Some species of Christians are very quick to dismiss everything that challenges their beliefs as "demonic." Near Death Experiences are "demonic." UFOs are "demonic." After-Death Communications (of which I've experienced several that have greatly reinforced my religious beliefs) are "demonic." Reincarnation research is "demonic." Seldom do these folks have a deep understanding of the subjects they dismiss.
The UFO phenomenon is complex and multi-faceted. Very few in the field think in terms of "aliens from other star systems" anymore. "Demons flying around" doesn't come close to fitting the phenomenon (or more properly the "phenomena" since there is no single "UFO phenomenon"). The phenomenon definitely does have, however, a "psychic" or "spiritual" aspect.
My guess would be that the UFO phenomenon might be used by demonic forces to steer people away from the truth. All areas of "weirdness" such as this can easily become obsessions and even quasi-religions. Many researchers have warned against becoming too deeply involved. This is the sense in which I believe UFOs - and lots of other things, including Facebook and Twitter - might be demonic.