Are the UFO's a sign? I believe they very well could be. Missler and Eastman wrote an excellent book called Alien Encounters. One of the most important aspects of it is found in the below passages with emphasis on Daniel 2. Something is unique about the final kingdom in that "they will mingle with the seed of men". This, of course, brings to mind Genesis 6 and the reason for the Flood, a co-mingling of the daughters of men with the sons of God--the fallen angels. The UFO's could very well be the coming minglers to rule with the seed of men. Many of the abduction stories involve an interest in the seed of men/women. The Antichrist after all will be the seed of the serpent--Satan. His identity will be hidden until after the Rapture. Indeed, this will be the worst period of human history, simply beyond our current comprehension.

So the LORD God said to the serpent:
      “ Because you have done this,
      You are cursed more than all cattle,
      And more than every beast of the field;
      On your belly you shall go,
      And you shall eat dust
      All the days of your life.

And I will put enmity
      Between you and the woman,
      And between your seed and her Seed;
      He shall bruise your head,
      And you shall bruise His heel.”

Whereas you saw the feet and toes, partly of potter’s clay and partly of iron, the kingdom shall be divided; yet the strength of the iron shall be in it, just as you saw the iron mixed with ceramic clay. And as the toes of the feet were partly of iron and partly of clay, so the kingdom shall be partly strong and partly fragile. As you saw iron mixed with ceramic clay, they will mingle with the seed of men; but they will not adhere to one another, just as iron does not mix with clay. And in the days of these kings the God of heaven will set up a kingdom which shall never be destroyed; and the kingdom shall not be left to other people; it shall break in pieces and consume all these kingdoms, and it shall stand forever.