OK, my two cents worth...
I believe in the last chapter that Oliver mentioned that he talked to David many places, but David only talked to him in the ONE specific place that he took Frank - And that David stopped responding to him very recently. My thought is that David is physically near that location - alive, or was until very recently. Oliver said that you had to yell to get David's attention.. so, buried alive? or imprisoned somewhere near?
Just a thought.
Thanks,
Terry