Okay, I must be nuts. After 5 times through this mansion, I'm always missing one lone red key at the end. I get to room 16710 and can't open that last door.

I'd sure appreciate someone sharing the secret of where all the red keys are to be found.

I'm not 100% sure this is right, but I think you have to play the top part of this mansion correctly in order to end up with the extra red key.
8-)

Hmmm ... thought I had, but I'll go back through my saves and look.

I don't have a clear answer yet, but at my last save in room 16110, before going down the pole to the bottom area, I already had one red key. I also checked that complete lower area again and there is indeed only one red key for two red doors.

So I am also wondering if these involves a Jack-stuck. :-/

Actually it is not a JS but it seems you cannot open the red door to the small treasure in the final area if you don't have a red key with you from the saving point above.

Of course!!  ::) Was a week away ...

At last I found it. The question is not: where is the missing key?! Between save 2 and 3, in the area with the gold/silver puzzle, the trick is NOT to open the red door with the red key you find nearby. Because there is really no need to.

Is a stunning puzzle, in my opinion. 8-)