I've read of patients having 'things' left in, and while that is alarming, it also gives an example of how the human body protects itself. It usually forms layers of soft tissue around the foreign object to protect itself.
While i believe that using anything without a radiopaque strip to pack away organs during a procedure is asking for a disaster, it really would not be that hard for something like this to get lost. At the time, the towel was probably soaked in blood and blended in with the surrounding 'red' areas. Also, depending on the patient's size, putting a towel in the abdominal cavity is like dropping a rock in a barrel full of water.