Employee policy handling lax work effort that may lead to internal theft

A medical practice recently had two instances of what appears to be cash theft by an employee.  The staff in the reception area that handles the cash boxes had been quite lax for some time now when it comes to the security of their cash.  The practice wants to have zero tolerance but was unable to determine who is taking the missing cash. The practice recently adopted the following changes:

1.    The safe combination has been changed.

2.    New cash boxes and keys have been purchased. 

3.    New locks have been put on reception area draws where the cash boxes are kept during the day.

So what is a good policy regarding theft/missing cash?  Obviously if a theft can be proven, immediate termination must occur. But what about sloppiness or carelessness that could lead to a theft? How do you handle that?

I like how one physician office handles it – a policy called automatic Corrective Action - Decision Making Leave – used when employees don't lock draws; leave cash unsecured, can't identify where cash has gone due to sloppy work, etc. Under this policy, employee is sent home for a day without pay. It sounds much better than “Suspended Without Pay.”