I wonder if David's wife is sitting in a cell with Belle now? .
That would be my guess.
Logical, though it would be interesting to know who's providing Regina's muscle. But it is perhaps too logical. David's vindication would seem to require Regina's exposure, and this would be rather premature. Yet a permanent cloud thrown by suspicion of murder is too likely to finish David/Mary Margaret.
Kathryn's happy ending involved ditching Prince Charming, then getting golden boy back. The Queen's curse shackled her to David. Leaving Storybrooke was self-liberation. In other words, there's a chance that Kathryn has started her happy ending by getting out of Storybrooke, back to Fairy Tale Land. The car's left there because it belongs in the real world.
There's also a chance that it was Gold who engineered the disappearance precisely because the investigation will ask "cui bono?" Then, it could lead to Regina's downfall. At least, so he might hope. The mechanics of how he might do this are pretty vague, but this is a fantasy.
The arbitrariness of fantasy leaves us more or less at the whim of the writers. We can hope that at least the characters aren't as elastic as the plot restrictions. Regina speaks of Kathryn as her friend. Regina was the Queen, who had no friends, but is the Mayor truly still the same? Rumplestiltskin has been softened (or deepened, if you will,) and he is continuous with Gold. But in a certain kind of popular fiction, it's women who are truly evil. It would be disappointing if Once Upon a Time fell into this nasty little stereotype.