Cameras still need to be monitored which costs money. Plus sleight-of-hand and other tricks are a thing, so you'd probably still need to background-check any contractors and maintain a strict chain of custody over access to the cube and then recount and re-check to make sure none of the currency has been substituted by lookalike fakes. Police still costs money.

If there is no reasonable way for the public to notice, why not make everyone's life easier by using fake money? That would be easier and cheaper.

Leaks can be dealt with by the legal system (pay people decently and make them sign an NDA in exchange not to disclose the bills are fake) which is much easier than actually keeping track of 1M of currency.