do you see the window on the side off the image that one. i also double check whole dungeon and found nothing :[

Yes, it does happen. Much like sunken crypts, the larger the mistlands area, the more likely you are to find more than one infested mine. And yes, some of the mines you will leave with 8 cores, others with 0 or 1. Hang in there, fellow gamer; the rewards the cores will unlock for you are way too sweet to miss out on. :-)

listen to enemy sound cues. if you can still her some you missed a hidden room. otherwise its clear.

First of all, it does happen, just like some chambers or crypts can sometimes be empty or ridiculously small.

Second, sometimes doors are hard to see and require you to break the wall. If there is a window to a room you haven't visited it's a hint that there is a hidden door somewhere.