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Another option would be are they are seeing something at the limits of perception, out the corner of the eye, or in a dimly lit room, and they are simply misinterpreting what they are seeing because they have an emotionally charged reason for wanting to see something.  How can you “know” that a disembodied presence making a chair rock is one of your loved ones sending you a message?  Obviously it’s not reason or logic at work there but emotions.