That is just the thing, spending the money is theft, irrespective of any steps you have taken the trace the owner of the money.. If the time comes and the owner wants the money back and you cannot pay it back, you are on the hook for theft..You have taken all reasonable steps for the money to be returned to the owner but did not succeed, You then spend the money with a clear conscience, no need to bother yourself about legally claiming it. You'll deal with the problem of the money being demanded back or SARS when it happens.
As others have said its not your burden to carry as long as you know what to do when they coming looking for the money, there is no way anyone can try to pin it as theft on you.
That's why I am asking about, if there will ever be a scenario where you can have a legal claim against that money..