Keys, pens, socks, brain cells and various other related items are said to disapoof, possibly coming to reside in the nether regions where such things go to work, play, spawn, and live out contented little lives.
The "poof" is reminiscent of the sound magicians (or wizards, witches, etc.) make when they perform some sort of magical act, drawing attention away from the object so that it can be discretely removed.
If it is so desired, parents might tell their children that "Fluffy" or "Fido" disapoofed, though the age-old explanation that said cat, dog, mouse, or goldfish merely swam or ran away is more foolproof. (this is not very effective on many adults either, so caution must be used)
"Timmy, I know you miss Rocky, but he disapoofed last night, and is probably running around chasing squirrels with all of his other doggy friends right now."