Ken - HCEG owns the land you refer to. And very desirable it is too - when Tom Doak showed some of us round the nextdoor Renaissance Club, he eyed it covetously!
If it were up to them, i don't think the land would be developed. After all, why would you want to dilute the exclusivity of your club (and probably destroy its character?) by doubling its size? I think they have about 700 members, but only a small portion of those play regularly so it is not as if the course is congested.
If, however, the laws of the land were to change such that an all male club ceased to be legal, then building a second course might provide them with a means of seeking to adapt to the new situation.
I raise the latter possibility because there are already noises that the government plans to outlaw clubs discriminating between male and female members (the status quo with many/most clubs where women pay lower dues for restricted access). This law does not I don't think extend to Muirfield which is men only, but you could imagine that a next step might extend to all male clubs.
To your question of hidden bunkers: there are too many for me to recall them all, but yes there certainly are quite a few bunkers you can't see. One example would be the central bunker short on the ninth fairway. You can see it in the photo i have shown, but not if you are standing back in the fairway hitting your approach. If memory serve a number of greenside bunkers are not visible - but we are not talking "sneaky" hidden such as you encounter on TOC.