I don't think it's terribly unusual, though I couldn't say exactly what the nature of the phenomenon is of course. I don't think I'd worry about it unless you are experiencing other symptoms, or it's causing you problems.

How solidly are you seeing her, out of curiousity (if it's ok to ask)? I remember seeing a deceased friend on the bus, but his image was not solid. Heard him a couple of times too. I think I simply missed him.