From Alfred Hitchcock Wiki

There is a possible cameo by Hitchcock in The Man Who Knew Too Much (1934), approximately 31 minutes into the film.

As a bus passes by, two men walk from screen right to left. The bus has an advert for the "Isle of Man" (the location used in The Manxman), and Hitchcock would also make bus related cameo appearances in the following films:

• Number Seventeen (1932) - unconfirmed
• The 39 Steps (1935)
• To Catch a Thief (1955)
• North by Northwest (1959)

The man's face is only visible for a few frames of film...