You may want to retain an attorney to immediately pursue an injunction stopping him from accessing the box, until you can pursue discovery to determine what's in it. The divorce case would have to be reopened.
The failure to disclose assets is grounds to have the equitable distribution part of your divorce case re-opened. This is - potentially - considered fraud. You should consult with a local family law attorney to learn more about the process.
James F. Feuerstein IIIBankruptcy AttorneyTavares, FL ·
Mr. Mullaney is correct, failure to disclose is a serious matter and could cause the entire equitable distribution agreement or order to be set aside. Talk with an attorney about his as soon as possible.
Before doing anything else, you should contact a family law lawyer. There are serious consequences for failure to make full disclosure, one of which is reopening the case. It's difficult to discern from the question, because I can't tell whether the case is open or closed. It makes a difference.