User blog comment:Broman178/Merlin's ability to sense magical objects./@comment-95.119.159.202-20120810191050

Maybe the ability to sense magical objects is somehow destiny's call; maybe in 4x03 and 4x09 things were destined to happen without Merlin interfering, and that's why he couldn't sense those objects. Whereas in the other episodes, people tried to mess with fate/destiny and Merlin was "allowed" to intervine and therefore given the ability to sense those objects.

Just an idea...