This incident took place in 1998 in an old monastery in Nijmegen, The Netherlands. I lived in this old monastery that had been redesigned to be a crisis home for youths that were having social problems, like runaways, homeless children and so on.
Every day people there would get chores to do in the household, like cooking, etc. On this specific day it was my turn to mop the floors on the ground level. These floors were L-shaped, with the short leg of the L being the part behind the front door of the building, and the longer part being the great hall, with the different rooms, where we lived.
When I was mopping just behind the front door, I stood with my back toward the great hall and suddenly felt like I was being watched. Dreaded, I turned around to see who was watching me, but the only thing I saw was a black, shapeless shadow that floated in mid-air and dove away behind my back, going in the direction of the front door. The door just behind the front door moved on its hinges, but nothing was visible anymore.
I was terrified and ran to other people, just to see that I wasn't alone. Later, I talked to one of the nuns who still did community work there (the normal staff were social workers) and told her this story. She told me that she did see similar things in this building in the past, too.
She also came up with the fact that multiple nuns died there, and that in the Second World War the Hitlerjügend was stationed in the building. Many a night have I thought about what I encountered that day in the hall, but I am sure it wasn't a "good" entity.