[Koha] Self Check In

Hi, i work in a university library where we use koha 3.4.5 on debian squeeze. Lately the we have been experiencing situations where items that have been check out and not yet checked in appear to have been checked in, in the user's circulation history. An sql statement to retrieve items checked in on that day does not include the item and the catalog record of the book indicate that the book is lost. What could be the issue? Regards, Mugoh Muriuki.