There are two main types of double eyelid surgery: one is suture embedding, and the other is external surgery. The other methods are all hyped.

The scars left by double eyelid surgery are the traces of double eyelids later, so anyone who is not a professional plastic surgery expert will not be able to tell that double eyelids have been done. It won't leave any noticeable scars like you said.

Reference: Weifang Medical College Plastic Surgery Hospital