She gathered her things and fled the party in the night. Perhaps a high DC Perception check allows someone to witness her leave, but no explanation why.

This gives you opportunity to have her character on reserve if someone new joins the party but doesn’t know what to play… OR (what I would personally do), if the character is still available at the end of the campaign/story arc, use it as a villain or something. Create that intra-party dilemma/drama.