Are there cases of pancreatitis which don't have a discernible cause?

Yes ("IDIOPATHIC"): App.10% of cases of acute pancreatitis have no known cause. Most people who suffer one episode will not have any recurrences. However, a second episode of idiopathic pancreatitis should lead to a more diligent search. The most useful tests in this circumstance include endoscopic ultrasound and ercp. Many of these cases are believed to be due to small gallstones ("microlithiasis").

Pancreatitis : Yes, there are called "idiopathic" however in most cases, a pancreas expert is able to identify the cause of pancreatitis.