Three thoughts on this very interesting subject:
1. Regarding the OP, it is quite possible these "Unidentified Aerial Phenomena" are next generation weapons systems, and this disclosure is disinformation, just like the disinformation used to conceal the development of the U2 spyplane, the hypersonic A12 Oxcart and SR-71 Blackbird spyplanes, the HaveBlue prototype for what became Senior Trend, the F-117 "Stealth Fighter", which operated in complete secrecy out of Area 51 for the first few years of its service life, and other experimental aircraft such as TacitBlue, the rumoured Aurora spyplane, a very appealing prototype fighter concept from the mid 1990s the name of which I forget, the YF-22 and YF-23 Advanced Tactical Fighters and their "X Plane" prototypes", and of course the F-22 and F-35 themselves, and most recently and famously, the stealth Blackhawk helicopters developed at Groom Lake (Area 51) used in the raid that killed Osama bin Laden (one of which was unfortunately lost during the mission). There are no doubt further experimental aircraft in development. And Russia has been working on an unmanned, hypersonic drone/missile hybrid.
2. If these are not advanced weapons, I think they are demons, because we are so far away from the nearest habitable planet, and projects like SETI have as of yet not detected any radio signals indicative of alien intelligence, which seems very unlikely (we would expect at least some accidental transmissions as a result of leaky electronics). Now, why do I think these UFOs would be demons? If we look at the writings of the early church fathers and of the apostles, it becomes very clear: Paul refers to the devil as "The Prince of the Power of the Air" or equivalent phrases. And in the Patristic writings, if we look at the Lausiac Histories, and within them the Sayings of the Desert Fathers, and related literature such as the Ladder of Divine Ascent by John Climacus, we find many instances of demons manifesting apparent flight. Often they would tell a solitary monk (or anchorite) his sins had been forgiven and that God had granted him the power of flight, in order to lure him off a cliff, resulting in the poor monk tragically splattering in the canyon below. Following these grim incidents, many cenobitic monasteries (formally organized abbeys or monasteries where the brethren are under the rule of an abbot and the supervision of novice masters, priors, and other leaders within the monastery) adopted a monastic rule that discouraged or prohibited the brethren from looking into the sky.
There are also reports of experienced solitaries and other monks and nuns, who became true spiritual warriors, with great abilities of discernment, tending to avoid looking into the sky, because they had gained the ability to perceive the bodiless powers, and knew the dangers of looking at demons.
As recently as the 19th century, we find this instruction repeated in a famous instruction manual for monks, written by Vladyka Ignatius Brianchaninov, a Russian bishop (who like all Russian bishops, started out his career as a simple monk, and then became a monastic priest, and then the abbot of a monastery, before being entrusted with a diocese - note that Russian parish priests are mostly married men with children, this applies only to bishops in the various branches of the Russian Orthodox Church). Vladyka Ignatius stresses that monks should look to the ground for reasons of humility, as a symbol of their penitent lifestyle, but he also does mention the dangers of looking towards the sky.
Finally, we have yet another Russian Orthodox monk, only this monk was an American, who converted, following a life of homosexual debauchery in the North Beach neighborhood of San Francisco in the 1950s. In the 1960s, Eugene Rose was called by Jesus, and felt the need to repent; he was specifically saved by a Russian Orthodox church in San Francisco, and by the famous bishop Vladyka John Maximovitch, and he was tonsured a monk, taking on the name Seraphim. He later founded a monastery north of Redding, California, which is still thriving today, and in the 1970s, before his death, he wrote a famous book entitled Orthodoxy and the Religion of the Future, in which he analyzed from an Eastern Orthodox perspective the various cults which were thriving in the late 1970s, such as the Hare Krishnas, Transcendental Meditation, Scientology, and the UFO movement. He was completely convinced of the demonic nature of UFOs.
Based on these ancient testimonies, I believe firmly that any UFO which is not a military test aircraft or a peculiar weather phenomenon is certainly demonic.
I should also add that the rules against looking into the sky were intended for monks engaged in a life of continual prayer and extreme ascetic discipline; they were not intended for laity. I myself love looking into the sky and seeing a beautiful sunset, and mercifully our Lord has kept the veil closed for me, so I have not seen any angels or demons in the air. This is particularly fortunate as one of my great loves in life is aviation, and I think piloting an aircraft would be extremely distracting if one was witness to what many monks and nuns have reported seeing in the heavens over the centuries. At a minimum, one would have to fly on instruments.
So, since I strongly doubt anyone in a monastery is allowed to use Internet forums, if you are reading this, I think we can assert that it is safe for you to look into the skies and view the beautiful sunsets and other aspects of God's creation without fear. This might change if you decide to enter a monastery, but even then, such a rule would be something that would be given to you by your abbot, and I would expect the majority of monks do not see celestial phenomena of the sort warned about, and there are certainly many monks who do look into the skies safely.
The specific cases I mention are extreme examples recorded in the annals of Christianity; they are relevant in this thread only because we are dealing with a subject that is particularly extreme.
