On Monday morning this week at 6 am and last week at 4:30 am. Mind you we have all windows cat-proofed so I think s/he must be quite skinny to fit in (very fluffy so hard to tell) we have a cat who doesn't care at all but this intruder keeps hissing at her and screaming quite loudly. Won't let me approach either. I don't know any of our neighbors well enough to be able to tell if it's theirs or not but I've put up notes in the building just in case no one has come forward for the moment a mystery visitor