My observations and opinion about the cross:
(I apologize if this has already been suggested as I haven't had a chance to read all the previous posts on the subject)
The cross is not very well executed, in terms of the carving. One arm is longer than the other, and one arm curves a bit more than the other. The cross is typically very symmetrical and all right angles. Even in folk culture handi-crafts, people that make these type of objects do it with a much higher level of precision, given the fact that they do it for so many hours of the day.
I believe the cross may have been made by the perp, or acquired by the perp in one of two places, prison or some kind of religious summer camp. It reminds me of something a person in jail would make, as many inmates find god in jail and are encouraged to participate in bible study and meetings of a religious nature. The other possibility is that the perp made this at some point in his/her life in a church camp type setting, where arts and crafts of a religious nature are often one of the activities available for attendees to participate.
I am not going to speculate about the reasons this was left by the perp at this point, although I do have some ideas.
However, I think there is a good chance that the markings were made by the perp, and hopefully they aren't some sick tally of his previous murders, although that would be something straight out of a thriller novel.