Typical - the one "priority" relative I had for the 1921 census is nowhere to be found. She's definitely alive in 1921. She's not with her family, and she never married. She definitely remained in England.
UPDATE: Finally found her - visiting an unknown family the other side of the country! I wasn't sure it was her, until I saw her mother's Surrey address in the (Employer)
Place of Work column! That detail confirmed it. But now I'm wondering what she was doing the other side of the country, given that she was in a 'vulnerable' condition. Hmm. Something for me to ponder tonight.

Found my career military grandfather, too. Stationed in Wiltshire. So that was a big surprise.