Thanks, I'm not seeing a separate staircase in those photos. Isn't this the loft area:
In the first photo you can see a bit of green just past the the double doors at the top of the stairs. That looks like it leads to the master bedroom (which is green). In the second photo, you can see the purple walls of another room that would be one of the girl's bedrooms. JMO.
You can also see the house plant that is in the loft through the door to the master bedroom in this photo:
All rooms seem to be on the same floor to me.
Thank you, that clears it up for me. I believe the loft is on the same level as the bedrooms. It is still possible CW hid the phone there but I think it is just as likely SW had a reason to place her phone there on her way to one of the bedrooms or that it ended up there during a struggle. All seem like plausible scenarios IMO.
ETA: I'm slow to post, sorry! I just saw others also came to the same conclusion about the loft.