My institution also rotates locks. There is the mindset that a copy of a key could be made, so we'd rather be safe than sorry. Fortunately, our lock can easily be changed by bank employees. We've also changed our policy that if both keys are not returned, we cannot close the box until we've been paid for 1 missing key, or paid for the drilling of the box if both keys are missing.
Of all the things I've lost, I miss my marbles the most.