Here's an idea, don't know what you'll think about this:
First of all, check very carefully it hasn't slipped under something/down the back of the drawers, etc.

Then step two is to call a family meeting and declare that since "someone" has taken it then "someone" has to pay for it - and dock both kids $10 to pay for it. They'll either fess up (the nonguilty party might dob) but they'll sure realise that there's a consequence should it ever happen again.

Just an idea, might be tough but it gives the message that a) it has been noticed and b) there will be a consequence if and whenever it happens. Of course it might not have been one of your kids but that's the risk you take in confronting this sort of thing.