Thanks for the chapter!
I bet the guys behind the mirror and the candlestick are looking for the eldest of three sisters who has a mark and were using the enchanted items as a means of finding possible targets. As for why probably some prophecy or ritual which requires a woman with those traits. That or those are the signs of a woman who would give birth to either a really powerful mage and/or one that specializes in all elements instead of just one and their sponsor wants such a woman at any cost. Will have to keep reading to find out one way or another but considering the methods it's for something foul and want to avoid attention from powerful figures. Not sure why chief wants them to investigate Eco though, unless pic wasn't her but her mother or sister, but might be related to the kidnappers.
As for why MC can use magic might be just a quirk, one part of being a special existence which is why also has a mark. Then again mark could be an indication that a woman can use magic and she's the only one who tried to cast magic or others were better at hiding could use magic after finding out. Being able to use magic without being identified casting it would explain why having crest makes it seem they bring blessings. Another possibility is all women can use magic too but usually can't because don't believe it, never trained it, and/or never recognized their magic as their magic and thought it was somebody else's magic or coincidence.