Even without any cpt codes entered, the patient balance due is being totaled by multiplying the # of encounters x the copay. Then if there are no codes, the insurance balance due is the negative of that total resulting in a zero balance due.
I have just recently seen it happen on 4.1.2(7) (in the last 2 weeks) I just thought it was an fluke and that the user created the problem but when I looked at the files (data) I did not see any thing that they had done. By the time I had finished looking at the files (~20 min) it was gone.