Used cat litter on my back stairs

I like my building. I have nice quiet neighbors that while they are not very good at shoveling snow (they always leave a thin layer on the ground) they do a decent job at landscaping; adding potted plants everywhere. I don't know who did this. Although I can narrow it down to at least 2 units I know have cats. It's icky. And this whole thing is not endearing me in anyway to ever have a pet.