So sorry this has happened to you. What a sticky wicket.
Well, differing opinion here - I'd ask the first guest who left early if she inadvertantly packed the blanket since it was in her room. She may confess that she erroneously took it and you haven't yet alerted the second guest of any issue. If she says "no", she left it at the foot of the bed, then ask the second guest if she inadvertantly packed it. Maybe she'll confess.
If neither fesses up, write to both to be on the lookout for it since while it isn't worth much, it has sentimental value to you - because it does (it belongs to you and you liked it there at the end of the bed and finding another one just like it won't be easy) and offer to pay to have it shipped back - which may actually cost more than the blanket's worth. You may still be able to send them your addy through the Airbnb system for a few days too. Sometimes a little guilt over stealing something will cause someone to send it back. At a minimum, every time they use it or look at it, they will recall that they are a thief and stole it from someone who opened their home to them.
Then flag the profiles of both girls and put in the other category that one of them stole a blanket from your home but you have no proof of which did the stealing. For the innocent one, it will never be an issue, for the one who steals from every guest a "little something", eventually this will catch up with her.
And then, you might consider, doing a room once-over after the guest leaves or ask them to hang on while you do a quick room check. It's tough when you have more than one guest who can take something and "blame" it on another.
Good luck