What should we do with the trust that the guard left with our father more than 10 years ago and has not returned to claim, in order to absolve our father's responsibility?

If you are unable to find the owner of the trust or their heirs, then give it away as charity on their behalf. If you find them later, inform them. If they approve of the charity, then all is well. Otherwise, give them the amount, and the reward for the charity will be yours.