Someone in stairwell

A nonresident in the building that I went looking for...letting them know I was coming for them. I was alerted by a resident that there was some one in the building that didn't seem they were a resident that was in the building and I went looking for them. First thing I did was check the stairwell and I could hear someone below where I was. I could smell that distinctive smell of Fetinal and yelled out that I was coming down and that they better get the fuck out of this building. You can't hear it on the video but I could hear them running down the stairs and exiting the building. When I got to the exit, I saw no one on the sidewalk. They were gone!