[Rain Man]

I don't understand why people are recommending talking to the employee. I figure all that will accomplish is that the employee won't like you and will wait to steal until you're looking the other way. It won't solve the core problem and will create another one.

Just tell the boss quietly. Don't do it anonymously because otherwise you place all employees, including yourself, under suspicion. It's the boss's job to terminate thieving employees and if it's a small business he/she won't mind doing it and will be grateful that you're looking out for the company. If he/she knows what they're doing, they'll set up a small camera, get the proof they need, and confront the employee without ever saying a word that you informed them.