Arthur is very secretive about his wallet. He doesn't like anyone touching it, least of all opening it. He keeps it on him at all times.
So Merlin was surprised when he found it on his dining room table the morning after he and Arthur went clubbing.
Must have gotten drunk and just chucked it there. He thought.
He picked it up, intending to throw it in the spare room, where Arthur lay sleeping, when he got curious.
He opened it and was shocked to find a picture of himself, accompanied by a little heart.
It matched his own perfectly.