Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If the situation is as explained in the above question, then that gold is a trust kept with you, so you aren`t liable for compensation unless you acted negligently or committed misconduct. As evident from your question, you haven`t acted negligently as far as keeping that trust is concerned since you have kept it in a place which you thought was safe and in the same room where you kept your wife`s gold. Consequently, you don`t have to make compensation.
However, if the heirs asked you to pay the value of that gold then you have to swear by Allah that it was stolen without any negligence or misconduct on your part. However, if you refrained from taking that oath, then the heirs can swear by Allah that they didn`t know that the gold was stolen and, then, you have to make compensation.
Yet, if this gold was found, then it is part of your late mother`s estate and you must divide it amongst the eligible heirs according to Islamic law of inheritance and it is unlawful for you to keep it for yourself and not tell the other heirs about it. And Allah The Almighty Knows Best .