The Gates/WMG

Leigh is a mummy; it's her heart in the box and it's what keeps her alive.
Leigh is a mummy; it's her heart in the box and it's what keeps her alive.
 * Maybe, though it'd have to be something fairly identifiable considering the thief seemed to know it was important right away (though whether it was valuable in it's own right or whether valuable as something supernatural is unclear). It also has to be something significantly damning and unusual to make her kill to protect its existence rather than simply be okay with it's return. It's absence seemed to affect her as well. Perhaps a hint can be from the story she tells about a woman who gives up something but becoming cold in the process. Marcus seems to take her actions as jealousy but perhaps she's in a state where she simply can't feel that sort of emotion.
 * Her story about a boyfriend ripping her heart out suggests that maybe she's some sort of a revenant, the thing keeping her alive is almost certainly her heart.

Marcus is not a Muggle.
In "Repercussions," there is a disc labeled Marcus Jordan when Devon opens the safe.
 * There was also one for the local gossip, Barbera, though and she didn't seem to be anything other than a muggle.