Similar to a "Clean Pinch" in as much as you go to wipe your rusty sherrif's badge after taking a forest and low and behold no evidence that you actually had a turd.
Fred - "It stinks so bad in there you must have left skids in your pants?"
When you have taken a turd and it is so heavy that it disappears and has already gone through the plumbing before you've had a look.
James took a big, BIG log one day only to find that it had disappeared by the time he had gotten up to take a look. The bowl was empty with no turd in sight. James excitedly ran to his mother - Mummy, I did a ghost turd!!!