If you have a safe deposit box with a bank (even in joint names with someone else) the bank will seal the safe. The safe cannot be opened until the bank or a notary has drawn up an inventory of the contents of the safe. The tax authorities must be invited to the opening of the safe. They will receive a copy of the inventory.

Insurance companies in Belgium have a legal obligation to notify the tax authorities of the existence of any insurance contracts you or your spouse have subscribed for any tangible movable assets ; those can be te furniture, jewels, art, the car, etc. This gives the tax authorities an idea of the value of your estate.