There are a lot. Also you don't have to "run around" anywhere, if you warp to a zone after you have it you will see all locations in the zone with secret doors. Same with Dungeons you have already cleared, if you reenter them and look at your map, you will see any secret doors.
No need to recover old ground. Also the secret doors in old dungeons will have loot scaled from those old dungeons (read: poor), but the secret door in the zones can contain better things, including set pieces and skill books (Cradle of Summer has both).
when i was in the thresh dungeon in the first zone (Dorguatha? ) i noticed 2 mobs i hadn't killed below me and i was missing one lorestone. i assumed that there was a hidden door in the dungeon and later came online to see if there was, peopel say there is one in there, i havent gone back to check. might not be important unless you do a pure finesse build or a ?/Finesse build with points into faeblades cause the lorestone buff for that dungeon is like 5% to faeblade damage, iirc.
" I eat dead people " - Dovahkin,Skyrim Xbox Gamertag : XXDemonichateXX