Regarding 401(k) accounts, the whole purpose of designating beneficiaries is to pass along the money when the account owner passes. In addition to the primary beneficiaries, there are also secondary beneficiaries that are usually noted by the account holder in the event that the primary beneficiary either passes at the same time or prior to the account holder.
If beneficiaries are not noted, then the account funds go into the estate and through probate. The fund company does not keep the money.
It sounds like what you could be dealing with is a pension. With a pension, there is generally a clause to continue the pension payments to the spouse if the former employee passes first, but it doesn't go any further than that. In the event that the former employee outlives the spouse, the pension also ends at death.