What's your risk? Do you have perimiter alarms that will sound if the door is breached after hours? If another employee found it would they be able to get to sensitive data during hours?
How big is your building - what would it cost to re-key?
Does the employee have an idea where they lost it? If it's in a snow bank along the snowmobile trail, why bother. If it was left in the bank parking lot then that's a different story.
Does the key or key chain identify the key as a bank key?
If the key says XYZ bank on the key or fob and was dropped in the parking lot of the main office I'd re-key regardless of the cost. If the key was lost away from the bank and there's little chance anyone would know that it's a bank key, why bother?
In short - there are too many variables for a single answer.
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If you approach life with pure logic you can avoid almost all of the fun.