This is exactly what Mr. Chadwicks ex-wife and lawyer did. They found NOTHING! They even hired an ex-judge who found NOTHING! They tried everything they could to find the alleged hidden money short of an FBI investigation. They found NOTHING!A forensic accountant is who you call if you want someone to investigate whether she has accounts somewhere that she is not disclosing. I do not necessarily think they would be able to determine exactly where this mystery $30k went. If she took the money out 4 years ago, well I can assure you that you have spent at least $30k in that time on household expenses, personal expenses, necessities, and luxuries in that amount of time as well. But if your primary worry is that she has some huge savings account hidden somewhere that she is hiding from the court, a forensic accountant is your best chance to find it.
And LD's point is very worth noting - even IF you find a hidden account containing the money from this 401k, if she pulled it out from an account she accrued prior to the marriage, and kept it separate all this time, it isn't marital property anyway.
Mr. Chadwick went to prison for 14 years anyway.
If the judge in OP's case "thinks" the stbx has hidden the money, the same thing should happen to her until she produces the money.