It is very likely that your surgeon used sutures that dissolve, which is why they are gone. It is very unlikely that you “healed over” your stitches. I recommend calling your plastic surgeon to confirm that type of sutures he used. Please keep in mind that it is best to have a go to your follow-up meeting with your plastic surgeon. Some stitches may be dissolvable and over a period of weeks to months disappear completely, other stitches may need to be removed.
You should check with your doctor's office as only your surgeon would know what sutures were used.Some stitches dissolve and do not need to be removed.
I would ask your plastic surgeon's office what type of sutures (stitches) were used in your chin implant surgery. It is possible they used sutures that dissolve over the course of several days and this could be why none are visible now. You should not have healed over your sutures.