I am in the same position and I understand that the result must be ?.

Solution 1: ( "!" are the positions that one can safelly call game.open. "X" upper case is the first guess.
x 2 3 x 2
2 ! x x 2
! x 3 3 2
! ! ! 2 x
! ! X 2 1
In this case we found one possible solution.

Solution 2:
x 2 3 x 2
2 x ! x 2
! ! 3 3 2
? ! X 2 x
? ? ! 2 1
One mine in any of the ? will also be a solution, so we have 3 more possibilities.

As we end with 4 possible solutions the return should be "?"