Shadow outside my door

I've just moved in to my new apartment. It's a very old flat, were a single lady lived and died just couple of months ago. I called my mother one say to visit me and we were sitting and talking in a living room for a long time. Around 1am we heard noise outside my front door as if someone was trying to get in. I walked to the door and looked through the peep hole and saw nothing, but then a black shadow quickly passed right next to my door. And I've been seeing this shadow almost every night ever since. I never see him at a day time, it only appears at night. I need you help guys to determine wheter it's an actual ghost or just something is wrong with my peep hole. Thanks!