It's the manager's problem, they signed it.
£2,000 went missing from where i work once but nothing ever came of it. Everytime something gets lost or stolen they throw the 'investigation' word around to scare people but they never actually make it come to anything.
They have to be careful i guess as they can't go round accusing people of theft.
If it was me i'd go to the manager and casually say, 'so whatever happened to that investigation?! Did the money tunr up in the end, etc etc