Uniform Stealing Board

Old movie from set during World War 2. A kindly British governess named Ms. Froy (played by the recently departed Angela Lansbury) is actually a member of the resistance transporting intel that will help the Allies squash the evil fascists, but goes missing on a train. At 40:50, the heroes think they've found her, but it turns out to be a different lady in the same outfit, who has taken her place to cast off suspicion. Later, at around an hour and 8 minutes, the real Froy is found bandaged and gagged, wearing only a slip. She reveals that the impostor from earlier stole her clothes for a disguise, and takes them back after the impostor is handgagged by the hero.
https://www.youtube.com/watch?v=Ql8wD_Jwnk0

A really terrible remake of Hitchcock's 1939 classic. But Angela is much more attractive stripped to slip than Came May Shifty would have been.

The Jessica Fletcher in a slip scene begins around 1:07