Reuven lent Shimon money indefinitely, he said "pay me back when you have extra money". It's been a number of years, and Reuven has never brought it up, and would like to forgive Shimon. He does not want to notify Shimon, because Shimon might look at it as a nice way to collect. I understand that being mochel in his "heart" works, but he could renege on the mechila, because it is divarim sh'b'lev [See MaHarit brought in Ketzos 12/1].
What is needed to make it a total mechila, one that cannot be undone? Is mechila in front of one person, even a relative effective, or are there other requirements? Do the details - such as who Shimon is, and the amount of money - need to be spoken out?
Is Shimon, who probably forgot about the entire loan and was not asked to repay it, doing something wrong at this time? If yes, will Shimon be doing something wrong if there was a mechila that he is unaware of?