Board Thread:What If?/@comment-5674726-20141111204657/@comment-5674726-20141116110426

Protection from a distance could have potential but would Merlin know of threats to Arthur if he was not with him?

Him being able to sense threats is the exception, not the rule. He could sense the danger Arthur was in from a distance when he was near death in The Poisoned Chalice, and was able to see what was happening via the Crystal Cave in The Diamond of the Day but, for the most part, he doesn't appear to have a special sense for when there is a threat to Arthur, especially a non-magical threat, he needs to there to see it. The Gates of Avalon is an example of this; Morgana is the one who is able to detect the threat to Arthur's life and it is her vision that raises the red flag for Gaius and Merlin.

Arthur has needed a lot of on the spot protection from non-magical threats, ie. when Merlin secretly helped out in battle by dropping branches on the heads of attackers. He needed to be present to see the threat and react quickly so I doubt that protection from a distance would work in cases like that.