Remote bases .......those located in the Far East are the only ones which spring to mind...no secret bases in remote areas of the world.
Recent posting on the forum has the "missing" number down to 70 odd boats...this is not as unreasonable as one might think at first glance given the global travels and operational areas....the "unconfirmed kills" , damage which later proved to be fatal and mechanical failures which might occur.
Later type boats as sniper says problems in production, new methods of construction , training new crews, set against the background of the attentions of Allied bombing and tac. airforces , and increasing shortage of skilled labour.
Time ran out before teething problems resolved , fully developed they might have preformed well......sent out in April 45 en masse would have acomplished little.