It goes with the territory Tabitha. I don't know if you employ people but we make theft a Golden Rule. If you take something, whatever the value, and it doesn't belong to you, you get the 'Black Spot' aka you're finished. ( I could go on about ET's (employment tribunals) where we have had to cough up cash over this but that would make me seem cynical about overpaid silly servants and jobsworths so I won't)
If a customer ever mentions possible theft we offer to call the police straight away if they haven't already done it themselves. That soon makes people think of what they are actually saying/ accusing you (and your staff) of as they normally back-pedal with a screech of brakes.
Putting it in perspective though and I hate to say this, your staff won't share your morals and ethics. Why should they? They are probably fighting to survive on NMW and see your clients as 'rich' people just like insurance companies are seen as fair game even though it's the people who pay for insurance who pick up the tab. (no joke intended)
I think all you can do is check staff out as much as possible, it sounds strange but also get a 'feel' for your customers as many of them will be tea-leafs in a different way and stick to a black & white philosophy. If it ain't yours leave it alone, especially on a client's premises.
Best of luck. Don't let them get you down!
Phil D