Have you called the police? If not, I would do so. You need to make an example of this person, and this is definitely an arrestable offense- provided that you have proof.
The only way to properly check up on everything is to do it yourself. If your trust was violated once, it can be violated again. I'm not saying that you should become a mistrusting, suspicious person- but you should make it clear to your employees that your eyes are on EVERYTHING. If if you're not checking up on things as much as you should, the impression should be given that you see everything.
The only way to be 100% sure that everything is in order would be to put your eyes on everything, everyday. It may be a timesuck, but starting businesses tends to be an all-consuming activity.
"The first lesson of economics is scarcity: there is never enough of anything to fully satisfy all those who want it. The first lesson of politics is to disregard the first lesson of economics." Thomas Sowell