Over the last 4 years myself, my mother and father moved from England to Denmark and back again. The first place we moved into in Denmark was a large, open space flat on the fifth floor. It had a lot of large windows and due, to living in a large city, it was never really dark or shadowy. For the first six months it was only me and my father living there, as my mother was still in England finishing off various things. It was during this six months I began to see a medium sized black cat. It was always out the corner of my eye so I just ignored it. I have always had a very active imagination and assumed it was just that. I continued to experience that for the whole six months without my mother.
When my mother arrived she would often ask, "Did you just see that cat?" which led me to believe it was indeed a spirit or ghost. With my mother around it began to get stronger, we would see it properly. There was one time that I was in my room, talking to my mother who was in the doorway. There was a large, crinkly plastic bag in the middle of the floor as I had just been shopping and we both saw this black cat jump on it, causing it to make a noise and bend. I checked to see if there were any cat hairs but there were not.
Around two months after this I got my own cat, Kit, who was a large (huge!) tabby cat. She would often hiss at what we could not see or jump up and run as fast as she could out of the room, sometimes right out of the flat.
We soon moved into the second house, where we got a dog, Oscar. He would bark at things we could not see; sometimes Kit would hiss towards the same place at the same time. There were also several occasions when we would see our ghost cat chasing Oscar.
We were forced to get rid of out animals in the move back to England and now only see the black cat out of the corner of out eyes. I believe it could be my mother's cat Charlie, who ran away around 15 years ago when he was only a few months old. This would tie in with the time when we started seeing him in 2009 as cats don't usually live past 12.