This occurs when, during a normal fart in the standing position, a small grape-sized poop is rapidly ejected from the anal passage. This "secret grape" is undetectable, until one sits down, at which point, the grape is no longer secret.
John and Keith are walking down the road, Keith lets rip, the two friends chuckle. Later, they come to a bench, the two friends sit. Keith's face turns red and an ominous odour fills John's nostrils. The secret grape is no longer secret.
John - What's that smell Keith? and why's your face so red?
Keith - *stands up, squashed grape falls from trouser leg*