My 4 year old DD is not yet dry at night and wears Pull ups to bed to aid keeping her sheets dry. Every now and again, maybe once a month she will poo her night nappy. She uses the toilet fine during the day and there is no history of any issues with constipation or soiling during the day.
I have talked to her and asked her why she does it and she just responds with "I don't know". 
Has anyone else come across this?? I think it might be that she is being lazy and just goes maybe early morning. I think this as when I check her before I go to bed she is always fine.