For months after my mom died I used to "see" her out of the corner of my eye in public places. Then I would turn to look and it would just be someone that resembled her (same body type, or hair or shape of the face). I just assumed that it was due to the fact that she was on my mind. Later I started to think about the brain's pattern recognition system. The one that lets us see faces in electrical outlets and the grills of cars. It allows us to get a pretty good sense of something without complete information. And for all of my life whenever I saw someone from a distance, or in poor light or out of the corner of my eye that vaguely resembled my mother it probably was my mother. That shortcut to recognition usually serves us well. Its just that it doesn't turn off instantly when someone dies. Its that flash of pain you get when you remember "oh yeah she's gone" that makes these misidentifications memorable.
That being said, when you start having conversations with dead loved ones outside of a dream its time to call in a professional.