I'm in final edits. Three times my characters are having a heated discussion when they turn around and a person involved in what they are talking about is standing there.

I'm sure readers will notice. But all three fit in the scenes. Help, I need an alternative.

Here's the third:
ďAnd now Iím supposed to understand what Mark went through?Ē
Mom was staring behind me toward the door. I turned. Mark was standing there.
The other two incidents are very similar. Surely there has to be more ways writers can present a similar situation than this one.

Thank you all in advance whether you can help me or not.