arthur sends merlin knickers
Arthur is one of the viewers and somehow figures out it's Merlin (maybe his voice?) but can't be certain it's Merlin because the Merlin that lives in his building is really shy (or softspoken?? idk). He sends Merlin a pair of knickers to wear in the show and later finds out that it is Merlin because Merlin drops the knickers when he's doing his laundry.