mistake for - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Take someone or something for someone or something else, as in I'm sorry, I mistook you for her sister, or Don't mistake that friendly smile for good intentions; he's a tough competitor. [c. 1600]