I think most mothers of young children would remember when their father WASN'T home on Christmas. That's why I think she would come out and say he was home - not that she has concrete proof he was home.
I, myself, am trying to see how easy it would be for me to recall where I was for Christmas 10 years ago. Actually, I just did! I was working at the office on December 25th 1996 and being paid holiday overtime here in Texas - so I'm cleared. My proof is the copy of my time sheet for the last two weeks of 1996.
I suppose theoretically I could work my shift then hop on a plane to Colorado and get to Boulder just in time to commit the murder and be back in time for work the next day but, if so, wouldn't there be a flight manifest somewhere to prove that happened?
BTW, it's not uncommon for me to work on Christmas or Thanksgiving each year because I'm single and all my family live out of state so I usually work one holiday or the other so other employees with families can spend those days with their kids.
But Karr IS NOT a creature of habit. Even though he was married, we don't know what his habits were. Was he working at the time? Did he go "out of town" looking for work? Did he take "temporary jobs" out of town. Did he go to libraries? Did he and his wife go out? Did they have friends over?
Where is he getting his MONEY?
I'm sure people have been coming out of the wood work and contacting the Boulder police. Be patient.