If I went into a changing room and a nurse I knew promptly left I would probably not notice. If I did notice, I would assume they had somewhere else they needed to be at that time. If I then saw them waiting outside the door I would assume they were either waiting for someone else, had forgotten something or were going back to use the toilet.
I've also failed to notice a friend waving at me on the street until they waved their arm across my face, and definitely struggle to recognise colleagues in 'normal' clothes when I only ever see them in scrubs or uniform at work.
All of these things are normal and demonstrate normal social behaviour and the limits of human attention.