The magic eraseris a good suggestion. Nothing more abrasive than that. The magic eraseris really a micro-abrasive or even a polish. Another option is one of those white pencil eraser, not a gummy erasure either. The classic red erasercan stain and smear.
If it is a smudge of something, a cotton cloth dampened with mild dish detergent. Or a swab with rubbing alcohol. Dab at the area, straight in and out, and check to see if any was lifted by the swab. repeat with fresh swabs.
Use acetone at great risk. It is likely to smear any paint it touches and may deform the plastic itself.