You still have to keep track of it and keep in mind how much of the total sum is not yours. That is a mental overhead and inconvenience, especially when the alien sum is a significant part of the total.

It does. What if in worst case scenario, you on on hook for something like alimony or fine payment, your accounts are being garnished, n now this unexpected money is also used for those payments??

More importantly to me (OP of this thread here), and of direct relevance to this story, is the fact that the money in question does belong to somebody. Presumably they want it! So me just doing nothing about it already feels bad.

Bad in an inactive kinda way, but nevertheless, something that I, as a good human, should feel sufficiently bad about to warrant my taking action.