Encounter with a Mysterious Woman

The following is the true story which happened to me several years back. Truthfully, I cannot surely say this event was of a paranormal nature however it definitely has predispositions to be one and it is one of the most bizarre things to ever occur in my life.

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At that time I was living alone in a studio apartment. It all happened on an afternoon during a hot summer day while I was on my computer writing a research for university. As I was focused on my writing, I remember hearing someone knocking on the front door. Immediately I thought it must be either one of my two neighbors or the landlord since no one else has the key to the building. As I opened the door in front of me stood a woman that was neither my landlord nor a neighbor. She looked like an average woman in her twenties; she had long black hair and her clothing was quite regular. However, I was not familiar with her. This struck me as strange due to the fact that nobody can enter the building without the key. Also, there were no windows in the hallway. Without much thinking I brushed it off thinking one of the neighbors accidentally left the doors open when she asked me:
“Is this the nail salon?”“No. It is not.” – I responded. There was a nail salon located in the building next to the one where I was living in so her question did not confuse me more. On the contrary, what confused me more was that she was just standing there with half of a smile on her face for what seemed as an eternity. She did not say anything. After a while of thinking if I should just close the door, I decided to speak out:“There is a nail salon next door.” - Nothing; she did not reply, she did not speak at all anymore. I was pointing to the front door trying to explain to her where the nail salon is but she would not move.
“Do you want me to show you where it is?” – I asked. She just nodded yes. I climbed down the stairs and she was following me. As I went to open the front door, I realized the doors were locked.
“It’s locked.” – I remember I said it out loud because it confused me so much. The woman just looked at me as if she knew the doors were locked. I went upstairs to get the key and unlocked the door. She still would not leave! I pointed with a hand towards the nail salon but nothing, she was just standing there. It literally took for me to lead her to the front doors of the nail salon when she finally let me go. The truth is, she still had that blank expression of confusion with half smile while I was leaving.
I returned back to my apartment, sat on my bed and wondered: “what just happened?”
The next day I decided to ask my neighbors if they maybe let her in. Their answer was no, they did not.

Today, years later, I still wonder about the mysterious woman with half smile and how she entered the building managing to leave the doors locked behind her. As much as I retell this event to different people, no one seems to come up with a satisfying conclusion to the story. 

Thanks for a read!