Why should UFO mean alien spacecraft?

Unidentified Flying Object I don't see a mention of aliens in that definition?

The story so far, so i'm led to believe is that the turbine was hit by experimental robotic air craft, testing has been going on in the surrounding areas for a while now.
I have no details, just what I've read and heard today. perhaps tomorrow there will be yet another explanation...!

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ooops..... wrong forum!!!