flash a smile
flash (one) a smile
To smile at one, usually quickly and perhaps unexpectedly. She was sad this morning, but I think she's feeling better now—she flashed me a smile before she left.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
flash a smile (at someone)
Fig. to smile quickly and perhaps briefly at someone. She flashed a smile at me as if she recognized me.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.