Look carefully. There should be extra ref flags on the main map (remember to scroll to the sides of the surface world map). If I recall you only ever unlock 1 new level at a time, so if there are more than one level available to choose from, then that means one must be a secret level. Also keep some of those secret levels till later in the game. Some of them contain 'transfer creature' specials which come in handy later in the game - i think round about level 16 or so you cannot transfer a creature to the next level. To get round that you just play one of the secret levels where you can earn a high level vampire or reaper and then you transfer it from the secret level to the next campaign level.