I'm not really sure what you're asking for. If you're looking for a system capable of accepting payment without an attendant there are many. Here is just one example:
I was thinking of a system that would let you place a deposit immediately before rental (imagine a rental car in a public lot) and then automatically receive your deposit back when you are done with the car (which was possibly even left in a different lot). I'm thinking the car would use its sensor system to detect if any damage might have occurred and if not, it could trigger the return of a deposit as soon as the vehicle is locked.
Admittedly, I don't really understand the distinction of a "smart contract". I am only describing the function of the system that there was some confusion over.
Ok. What I'm saying is that the ability to accept and relinquish an automated security deposit does not require a decentralized technology such as ethereum or even bitcoin.
A machine (like the one I linked above) can accept money and dispense money at the discretion of the business rules that determine the appropriate criteria for these operations (via car sensors or whatever).
