You can get it sealed if it's small enough, which would probably be the best thing to do before returning it. Many insurance companies offer the stone chip sealing service for free and it takes less than an hour (and you don't even have to be at the car while they work on it - can be in the parking lot).

If you're on a corporate rate, does it include LDW? If yes, don't worry about it.

Are you renting on a credit card with primary coverage? If yes, don't worry about it.

If you do nothing at all, the check in attendant may or may not notice it. If they don't notice it, you got lucky, but if they do, expect a call from the Damage Recovery Unit. As 3Cforme said depending on what kind of vehicle the replacement cost could be much higher.