Meaning: mysteriously lost
Rewrite 1: fell into a sinkhole
Rewrite 2: slipped under the filing cabinet
Rewrite 3: warpediv2* into another dimension
Rewrite 4: inadvertently picked up by the elf doing my filing

*Here I mean “warped” in the sense as used in science fiction, in which spaceships can move into other dimensions by manipulating the fabric of space.

Comment: Although this cliché usually means that something – a piece of paper, a project file, or a project itself – has gone unnoticed or has accidentally left a place where you would see it to deal with it, it can also be a sarcastic response that says it was accidentally lost but means was actually intentionally set aside. This is a rarer use.

More reading about clichés
What I found when I googled “clichés”:
• Clichés in photography on, on Eschew Cliché by Mike Johnston, in The Luminous Landscape, a video journal by Michael Reichmann. Eschew Cliché is available on photo.net.