the cave is marked, the insides aren't, that's the point
it's not all about real life matters, it's also about the in-game course of how things can spell out when not playing in a linear way
I still don't get it. If you return to the cave, enter it, bring up your map of the interior, then some areas will by foggy--indicating you have not explored everywhere--or it will be filled--indicating you've explored everywhere.
I just really don't see how this doorknob thing is helpful, or anything but redundant.