Now, I find they have legs and they are walking out the back door. I'm at a loss of what to do. I would suggest it. They may not catch someone in the act, but the fact that they are there may be a deterrent. BTW, do not think that a dummy unit will work.
Employees have to know and see that both doors are being taped with time and date.
Most likely you do not see these attitudes as true for your firm but your employees may see things differently. One of the problems many businesses face today is that very few people come to the business world with any idea of how business works and what it costs to run a business. I'm not familiar with the markup on musical instruments. So, when they hear that you make whatever your markup is, even if it is lower that keystone, they think this is all profit and you should be paying them more. They have no idea of what it costs to run a business.