You post a public notice saying where the deposit boxes are being held at the original physical location, the company's web site, the local papers, and any othe

You post a public notice saying where the deposit boxes are being held at the original physical location, the company's web site, the local papers, and any other likely locations. If anyone shows up claiming to own one of the boxes you follow the same authentication procedures the original custodian would have used. Then you leave them in private to unlock the box and remove their property. At no point during the claims process should the temporary custodian attempt to open any of the boxes or observe their contents, as that would be abusing their power to search presumably innocent parties outside the scope of the warrant. If sufficient time (decades, at least) passes without any claim after proper notice is given, the property can be considered abandoned. At that point someone may attempt to open the box by force.