The purpose of charging for use of the machines is to cover costs and still make a marginal profit, not to snag rare coins.
Will you disappoint your employer if he gets the money he expects, not-with-standing the value of a few silver coins?
He is owed what he expects to receive.
I don't think that he has a greater call on the value of a "mistake" that do you. He's not in the salvage business.
If there is an ethical issue here, it concerns the person who used coins worth more than their face value, but if you don't know who they are, how can you be expected to share your superior knowledge with them, and offer them the chance to remedy their mistake?
As long as you replace them with legal tender, I say: keep them
As to whether or not anyone "fingers" you at some point, that's not an ethical consideration