Well Purkle...here's what I've been considering for quite a while...
I suppose at this point, this will be based on a container of some sort instead of a concrete basement or bunker type of structure, but I have been entertaining the idea of building a shed, or outbuilding of some sort directly over the top of the grow. In my opinion, if you build a shed over it, then your trapdoor options increase dramatically. With a cleverly constructed shed, you could have a false wall to hide an entrance; place some type of normally non-moveable equipment, such as a large table saw over an entrance; or even enter under a set of cabinets.
As for smell, I really think that you can even fool a K-9 with this approach:
Build-in a small passageway that is sealed fairly well from the grow space and the shed above, and place an O3 genny in it. Now, since it's not in the grow space itself, you should be able to run really high concentrations of ozone to kill all odors, and still not harm a damn thing in the grow room. If the coppers ever come looking for your grow, you know they'll have the K-9's with em. And I'd bet that if they see a concrete pad (floor of the shed) coupled with no indication from their K-9, ya might just fool em all....maybe I'm being naive here, but I'm sure others HAVE fooled em. Hell even the cops can't win em all.