Unless the foster care system is willing to cooperate and give you the needed information, I'd treat her like anyone else who moved and didn't leave a forwarding address. A letter dated October 1, 2013 lists the steps that needs to be taken before moving the person to "Address Unknown".
An alternative is to contact foster care and notify them that she is a member and request that they put her in contact with the church in the area she is in. They can use maps.lds.org to determine what unit that would be. That way they don't have to divulge information which might go against their rules.
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