This happens to me at the oddest times, and quite often. I could be in the middle of shopping, texting, walking, reading, typing, exercising, "just being", or ... you get the pic; but yes, this happens to me, too. It's a FLASH in my mind - just a split-second shot of what I KNOW was a dream from a while ago, possibly months or even years ago! I have no idea what causes this, but I've determined that it must be that one of our senses activates in a very precise way that triggers these random dream-shots. I also considered that it could be paranormal...still could be a scientific explanation, but scientists just haven't learned enough to call it science yet.