My ex sent me a similar letter. I'm not sure what it means in your case; in mine I'm fairly sure that she was looking for any chink in the armour, to force me to interact with her. Perhaps your ex doesn't even know what it is that she's looking for, other than to continue to interact with you, to provoke you into... .Something. She presumably knows that weird cryptic comments will get back to you, and that you may be angry or curious or confused.
Good luck with the NC
