everything reasonable (a ufo is not speculated to be a pancake) simply because the question is too general. a specific incident such as the belgium flap could have one of two explanations.
1- manmade: this seems unlikely but yet is cannot be ruled out. i am not privy to the r & d of the various orgs involved in this to say anything with certainty. the question here is how isolated can, say, the general academia be from a top secret research project? for instance, the stealth bomber; was what they were doing so revolutionary that there were not other similar research taking place elsewhere? wouldnt a radar gun manufacturer research the same shit that the stealth people did albeit perhaps with less resources? what about the atom bomb? what kinda secrecy was involved there and was the general academia left without a clue? the way i imagine shit to happen is some freak pursues a line of study, the defense establishment notices, locks shit up and moves to an undisclosed location after swearing all the players to secrecy (as seen in the movies). i find it unlikely that revolutionary shit can be kept a secret simply due to the collaborative nature of scientific research and advancement
2- et: rule out the 1st, you are left with this. a necessary corollary would be to accept that life/sentience is possible offplanet. here i can let my mind run riot (years of scifi novels)