Deleted Items from Deleted Item reappear

I hope somebody can give me a push in the right direction here. I have a user, that deletes email, contacts et al, from his deleted items folder once they age past a month. But he likes to keep the last months items in there for record keeping. However at some point down the road, either a random time period, or one that we can't pin down, they reappear. Why? And can this be stopped?

Thanks