An unexpected visitor

I was washing up last night, doing the usual routine, when I hear this knock on the window. A solid thump. I immediately look up yet nothing is there. Strange. I look back at my dishes. 15 seconds pass and then I hear it again. I concluded this was far to loud for it to be my imagination now. Was someone out there? Do I check? I'm living by myself so this is odd, if not a bit worrying. Real…