We recently discovered a bug in our payment gateway software. The amount of the transaction passed on to the client is less than the actual amount. The last time this piece of code was modified was 5 years ago. Since the payment gateway supports several other payment options, and the amounts lost were small, this didn't draw attention until recently.
The company has a tradition: whenever a developer resigns, he/she puts something bizarre (such as a silly comment or an easter egg) into the code. This tradition has generated some funny moments among colleagues. It is possible that a developer introduced this bug deliberately as part of this tradition, but we are not sure. Nobody who was with the team back then is working here now. Moreover, our company has a policy of deleting or anonymizing records of past employees, so we cannot be sure who introduced the bug.
How should we professionally handle this situation, and limit damages in the next meeting with the client? Is it professional/ethical to disclose that this "tradition" might have been responsible for this bug?