Is it permissible to conceal the loss of my father's watch, which I took out of necessity, until I have enough money to buy a replacement for it?

If you took your father's watch through usurpation, or as a borrowed item and it was lost due to your negligence, you are obligated to guarantee it. If it was lost without negligence, and you had taken it as a borrowed item, some scholars do not see a guarantee binding upon you, while others consider the guarantee obligatory. If you are obligated to guarantee it and are able, then give your father a similar watch or its value. If you are unable to pay, then inform your father, or write in your will that your father is owed this amount by you. It is recommended to hasten the writing of a will, and it is obligatory for anyone who owes a debt or has a trust.