Recently I noticed some marks on my apartment's wooden floor. They weren't there when I moved in 4 months ago. About a month ago I bought a vacuum cleaner https://www.davidjones.com/default.aspx?catid=881363&pdid=2120416, so I'm not sure if it might be the root cause. Can someone please take a look at these pictures and tell me if it's a serious issue or not, and what might have caused these? Thanks.