Don´t think we see the actual situation of the flat. We have to consider that GR arrived home on the 19th and i guess he stayed couple of days...maybe till they found the body. So for sure he touched the things as well and maybe even moved them.
And let's not forget that it wasn't even being treated as a crime scene in the week up to Christmas - JY's disappearance was being treated as a "missing person" until Christmas Day.