they hide in corners, in dark spaces, in abandoned places. there one moment, gone the next. leaving soot behind. can move fast, but never seen moving. is it just dirt or something leaving the dirt?

at night or in the morning when U put on the light in the barn, I hear a scritching sound and see some movements, but when I look closer there is nothing there apart from black grime. no matter how often I clean it away, these spots of soot are back the next day, often in different places. what is living in my house with me?