Transcript:
A child in a 'ghost' sheet stands exclaiming,"Boo!"
The 'ghost' approaches Violet and Patty and exclaims,"Boo!" Patty says,"That sheet isn't very white, is it, Violet?"
They walk off and Violet concludes, "Mrs. Brown must be getting careless with her washing!" The 'ghost' watches them go and shouts, "Boo!!"
Snoopy looks at the 'ghost' puzzled as the 'ghost' sits on the curb next to him and exclaims,"Rats!"
Original comic:
I guess they just know it's Charlie Brown under the sheet?