The one outside Mekkah only needs to face in its direction, because that's all he is able to do.
The one who is afraid of facing in its direction, for example during a battle, faces wherever he is able. He is excused, like the one who does not know the direction and makes an effort to find it, then prays.
The one who does not know the direction and finds no one to ask about that, makes an effort to make up his mind about the where he thinks it is, then prays. Note that:
- If he finds out later that he was wrong then he does not have the repeat the prayer, because the religion only orders to do what one is able to do, and all he can do is face in the direction he finds most likely.
- If he finds out during the prayer that he was wrong, or changes his mind about where it is, then he turns towards the new direction and continues from where he was in the prayer. This is what the Prophet's (may allah raise his rank) companions did when the direction changed from Jerusalem to Mekkah while praying.