Mine

History
I had been playing 1.8.3 for a while, I had built a big house and I was just starting building a big city (which I called Clifton). I was playing in creative mode and I discovered a load of black banners with blue + symbols everywhere. I was pretty freaked out as I was playing alone with no mods but just presumed it as a glitch.

Fast forward about a month to July 2015 and I was roaming around on a hillside very close to my house which I called Beacon Hill. I often crossed over the hill to visit a giant battlefield that I had built (despite being in creative mode I often like to walk between places). On the way back one evening I came across a small man-made cave with a 1x1x1 hole in the floor which was filled with a barrier. Behind it was a chest which was filled with about 10 of the same banners which had all appeared around the lake. On top of the chest was a sign saying 'mine'. I was in there for about a minute very puzzled until I heard some shuffles outside the cave and the sound of blocks breaking and all of a sudden Minecraft stopped responding and closed.

A few months later I was building a shop in Clifton when I heard a sound of a helicopter buzzing above me, the sounds went on for about 3 seconds before a break and then another 3 seconds. These sounds also happened around the same time of day in minecraft (sunset) a few days later, this time above the lake mentioned before (which was now called Staggar Lake) but a lot louder.

I have heard nothing of the noises since but the inside of my house burnt down soon after I saw the banners, with only the outside walls still standing. This has happened twice even though there were no fireplaces/lava anywhere.

conclusion
So whenever you see black banners with a blue +, they are warning you of unexplained happenings (fires, dogs disappearing, helicopter buzzing, etc.). The Helicopter buzzing has happened to one of my friends before and a huge skyscraper collapsed just after he heard it!