If you have a charge on your credit card still showing after the rental was returned that you
don't know what it is, then you may want to confirm with the car rental company that you rented from, what exactly the charge is - you can try contacting their customer service department or billing inquiries department.
It sounds like what's still showing may be the deposit that was placed on the credit card on the day you rented... especially if the date on the charge coincides with the date of pickup. I have heard that it can take up to two weeks for some banks to process a reverse of the deposit.
Once you contact the rental car company to confirm that it is your deposit, then you could contact your bank to see if they could give you an estimate on a length of time you could expect to wait until those funds are back and available in your account.
Hope this helps!