The "welfare" check.
For me, right now, it's the timeline. If it works out as suggested, the killer had a little over an hour to get in, kill, get out, without anyone seeing him/her. Not to mention taking time to write on the wall. I'm sure in reality world it could be done...but by whom? Me thinks it was the same person who was in the house before going to the gym!
Also, did he usually call his wife everytime he went to the gym? Man - what was this guys calling patterns?
The girlfriend (motive)
JMO --
Mel