A Haunted Mirror?

This is my first story. I've never really considered myself a believer in the supernatural...I've just always thought there must be a rational explanation somewhere. However, recently this belief has been challenged because something happened to me that I can't explain and it scared me a lot. This is what happened.

There's this old building where I live, just across the railway. I convinced my cousin to explore it with me; we thought it was all very amusing at the time. There was nothing there really, just rubbish, old machinery, bottles etc. However, I found a mirror, in all but perfect condition, I decided I wanted it... So we took it.

I'm a student of Fine Arts and for a while I'd been interested in painting on different surfaces, especially mirror and glass, but I'd been finding it difficult to source such material at a low cost. So when I found the mirror, I thought it was too good to leave behind.

I cleaned it and removed the frame. That night, I painted a portrait of myself onto the surface of the mirror, in oil paints. I left it to dry on a tarp, on the floor of my bedroom. It was fine when I turned my lights out at about midnight and went to sleep, I remember noticing it and thinking how good it looked. So, I was very shocked and alarmed [and annoyed!] in the morning, when I woke up to find it had large scratches all the way down it. The scratches moved down in a straight line and then pulled up again in a loop as if the instrument had been dragged back up. There was also a section where the scratch marks intersected in a diagonal/diamond-like pattern. There were also small prick marks of paint, as if whatever made the marks had rested on the surface for a moment. I'm sorry I can't explain that any better.

I say "instrument" because the lines were too evenly spaced, clean and fine to have been made with fingers. They were clearly made with something like a fork... But not a regular fork, something more like a small rake because of the number of lines that were grouped together.

I immediately examined my surrounding area, there was no paint smudged anywhere and no signs of anything else having been disturbed.

I checked the ends of all my paintbrushes (though I doubted they'd made the marks) and they were all clean. There were no instruments anywhere that could have made those marks.

I thought perhaps an animal, but my windows and door were shut, and my room is a new extension with no holes anywhere. I doubt even a mouse could have squeezed under the door. Even if it had, there was no fur or paw prints anywhere. And, it would have had to have had arms over 40 cm long to have made the marks in the way they were made.

I thought perhaps it was some kind of abnormality in the surface of the mirror and it had become visible as the paint settled into the surface. But it seemed completely smooth, and the marks extended beyond the painted surface. You could also see where paint had been transferred from one end to the other.

I have no siblings and I only live with my nan and mum. They're very serious people and don't play jokes on anyone. They love my art and would never sneak into my room in the middle of the night to do that to a painting of mine. I know it wasn't them. I also saw their faces when I showed them what happened; they were genuinely shocked. No one in my family did this. I doubt either of them (anyone for that matter) could have navigated their way across the sea of paint tins and jars strewn across my bedroom floor, in the dark, without knocking something over.

I thought maybe I did it myself in my sleep. But, I have no history of sleep walking that I know of... And again, I'm sure I'd have knocked something over. Plus, to find anything remotely matching the marks made I'd have had to go into the kitchen. That would have involved opening three doors, raking through the draws without disturbing anything else, and then returning whatever it was I used, washing it, putting it away and then closing three doors behind myself again... And all without my mum waking up whose bedroom door is always open and only three meters away from the kitchen. I feel like it seems "highly" unlikely I did that. Even if it was manageable, the closest instrument that could be found in the kitchen matching the marks on the painting, would be a fork... But like I said, they don't look like they were made with a fork, there were far too many prongs.

There's something else as well. My tarp was torn. It definitely wasn't torn before. It was a new tarp and I was being careful with it. I would have noticed. I tried to tear it myself to figure out how it had torn. I managed it but it took a lot of strength and effort to tear. There were also the same prick marks around the tear in the tarp as were on the mirror.

So what do you think happened? Is there something really obvious that I'm missing? Or, do you think this was something supernatural?

This happened a few weeks ago. I've since put the mirror in the shed in the backyard, and nothing else has happened. Though, whenever I go into my shed I now feel very scared and I feel like there's a strange feeling there. That might just be a psychological thing though because I'm still so confused and freaked out by the mirror incident.