Seeing a Ghost?

Any of the odd sightings I've had in my life can be rationalised away as mistakes, tired eyes, imagination and so on. I find this most difficult to do in the case of something I saw in my first decade.

I was around eight years old, or at any rate of an age when I was trusted to bathe myself but not to run the water at the right temperature. After running my bath my mother called me upstairs. I went up the stairs, not bothering to switch on the light since the bathroom light was on. As I neared the top, a shape (it seemed to be two-dimensional) fluttered through the house wall to my right, undulated quickly close to my face and then passed through the bedroom wall on my left. I wasn't particularly frightened; more surprised than anything else. I didn't even mention what I'd seen to my mother.

Above I've made a rough drawing. As you can see whatever it was didn't have much shape. The important thing missing is the fast undulating, wriggling motion it made. My house was a modern one in London of about the same age as me and it was built on farmland, so there's no 'old dark house' element involved.