The nighttime issue may be just what the first poster mentioned.. she is just too asleep to feel the urge and sleeps through it. I don't think that's all that uncommon. My son was the same way.. even at 9 years old, it was an occasional problem...
Also, bedwetting seems to have some hereditary patterns too. Were either you or your husband bedwetters? Ask your parents.. you may not even know it or remember it.
Heavy deep sleepers are just too deep in their sleep to waken enough to feel the urge. I did the "sleepwalk" potty trip with my son for years. He was daytime perfect, but at night...he was made to use the potty just before bed, and then I would "wake" him (actually half carried him out of bed to the bathroom) and stand him in front of the toilet. He would still be so asleep that sometimes I had to help him aim! As soon as he was done he would flush, yank his undies up, and practically run back to bed and was sound asleep again. He never remembered the next day having been gotten up to use the bathroom. This went on for a looooong time. On the occasions I would forget to take him that one last time just before I went to bed... he would usually wet during the wee hours. Occasionally he would come tell me (4 am or so) but as he got older (and probably it happened later in the night due to his growing bladder) he would not even realize until he was getting up for the day.
There is nothing wrong with using pull-ups at night for extra insurance, but I would highly recommend you try the "sleepwalk" potty trip instead. She might get a "complex" about wearing "diapers" at night if any of her school friends discover it.
Be patient with her during the day, too. She is going through a lot herself with the divorce of her parents. :)