This incident has happened to me quite a few times. It was the summer time and I was studying before the beginning of the new academic year. My room was just next to the staircase that one used to reach my house. The windows of my room would open towards the staircase. I didn't prefer sitting and studying in the room with the windows opened as staring at the night sky normally freaked me out. But since it was quite hot in those days I had no option but to keep them open.
My brother had warned me against keeping the windows open as he himself had some issues there but he preferred not talking about them. So there was always a kind of fear in my heart regarding the windows open and the staircase.
Then one day as I was busy studying, I heard some footsteps. As the footsteps grew louder, I could make out that it was someone that was climbing the stairs because if the footsteps were of somebody on the same floor their intensity wouldn't have changed. And also my flat was the only one occupied on that floor and my parents had already gone to sleep. So I was just waiting who could it be, who was climbing at the middle of night.
As the footsteps seemed reached my window and I couldn't see anybody by then, I went to the window to see who it was. But there was no one. I thought that I might be imaginning things. But it was the same story almost every night. So I decide to keep the lights in the staircase on at night so if anybody was climbing the stairs, I could see their shadow on the parapet. That night I again started hearing the footsteps so I went to the window to check out the shadow but I couldn't see any. And those footsteps sounded so real that I could actually feel the person standing next to my window but there would never be any shadow. So it led me believe that what I was experiencing was definitely a paranormal activity.
I stopped studying in that room since no one believed me and thought it to be my imagination.