If that's the case then I'm confused as to where the cryptocurrency comes in. A security deposit works just as well with fiat cash as it does with cryptocurrency and in your example there is no money exchanged at all.
I just don't see how ethereum (or even bitcoin) is useful here. If you make a deposit on a rental vehicle then the deposit will be returned when you return the vehicle as it was given to you. Why is a smart contract needed?
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:
What if you were unable to accept cash (for example, you are on vacation)?
Wouldn't it be useful to make a deposit on a car, right before you use it?
What hasn't been explained here, are what guarantees the depositor receives.