I'm sure this isn't foreshadowing anything.
Grace is a master of the ninja technique of "hiding the car when Dan doesn't want to draw it". Who is this Dan, and what does it matter if he doesn't want to draw cars? No one knows.
In all seriousness, that skull has to do with something I'd vaguely planned since the beginning, and I DO mean "vaguely". Grace's origins changed drastically, and I can't even remember what exactly that skull was originally supposed to represent. I'm guessing a variation of the same general thing, but I never wrote it down, and I can't recall specifics beyond, well, super vagueness.