I'm not ready to say that it's beings from another planet, but I don't have the dismissive, "Yawn," attitude, either. These pilots are seeing SOMETHING! To say it's nothing is being in denial. It's SOMETHING.
Alien? I find it hard to swallow. And yet, the other explanations make no sense to me either. Secret military projects? These are military pilots! They are already privy to top secret information. You brief them and say, "Look, this is something that can not be released to the public," and they shut up about it.
Not only that, if you believe it's military, then we're talking about some really stupid people running the project. Instead of operating somewhere that the machines can't be spotted, they fly them near U.S. Navy task forces where RADAR and fighters can pick them up.
I don't know. I'm hoping we find out during my life time. Doubt it, though.

"From our orbital vantage point, we observe an earth without borders, full of peace, beauty and magnificence, and we pray that humanity as a whole can imagine a borderless world as we see it, and strive to live as one in peace."
Astronaut William C. McCool RIP, January 29, 2003 - Space Shuttle Columbia