If Scott Peterson had not killed Lacy, but still had left for several hours on Christmas Eve, chances are real good that he would have said he was going fishing.
If she lived and was asked later, was Scott with you during Christmas she would say yes. Because Scott was known to go fishing a lot. Ten years later she might not remember the exact details of that Christmas...except of course her son would have been born and that would have marked it. Or he could have used golf as an excuse to see his girlfriend, and Lacy would still have assumed him "with them at Christmas", even if he was away for awhile, supposedly golfing.
So, you are correct that he could have lied to her by claiming to be doing something that she was used to, when in reality he was somewhere else doing something radically different.