HolgerDanske, I'm glad you said "In my view" there is no answer to it because it can't be objectively proved if harm has occurred if the person harmed is not aware of that harm.
Trust has been broken? the person in the dark may still of course completely trust, whilst the cheater also trusts the one left in the dark. So has trust really been broken?
People must have an opinion, because opinion is all there is to be had here 
Its a little like any contract. We have laws, and we have means of investigation, and we have judges and juries. But, first we must trust that the person making a contract with us will uphold the conditions, then trust that fear of consequences impels their actions, and finally if all else fails the law will ensure they either have to, or that we have justice. So, we don't blindly trust, but we have to trust all the same. Ethics do not impel people to behave. Something else always stands in place of morals to ensure justice. We are naturally suspicious of the motives of others. But actually, a far healthier way to be is to blindly trust, if we all did this, it is actually more likely that morality would guide us. Game theory can be applied to this to try and prove that trust/faith would lead to better outcomes. But all people would have to trust, otherwise those that don't would be advantaged, and it tips the scales in favour of suspicion!
Anyway, I prefer trust. It makes my happier!