Rules have changed over the years but a $6,000 400(k) will likely be forced out to a rollover IRA, which will probably be invested in a money market as the force out limit increased from 5k to 7k on 1/1/24 as part of SECURE 2.0.
I’d encourage your spouse/SO to track down the 401(k) and transfer it to an IRA even you need to jump through some hoops to do it.
Transfers to unclaimed property may still happen but I think they are rare.
A plan can’t distribute without consent if the vested balance is more than 7k unless the participant reaches the later of age 62 or the Plan’s normal retirement age, another exception may apply if the Plan is terminated and the participant is lost or non-responsive in which case they will most likely be forced out to an IRA or have their funds transferred to the PBGC program that’s a few years old and takes funds from terminated DC plan now in addition to covered DB plans. It’s also possible the plan has annuity option and an annuity might be purchased when the participant reaches NRA but this is somewhat rare I think for 401k plans and much more likely in a DB plan.
I think the only case where it might be transferred to unclaimed state property is if the participant is sent a check at NRA and it goes uncashed, at that point it might get transferred to unclaimed property.
There is a reporting database though SSA where terminated participants in retirement plans with a deferred benefit are reported and when you apply for social security if your in the database you get a letter that says “you may be entitled to a benefit from the XYZ Plan”. This can be hit or miss as plans are required to report but not required to remove so a lot of people who have long since been paid from the plan get the letter and forgot they were paid out. Oh and even if the plan removes you that’s hit or miss whether the SSA also removes you, especially on older forms that were paper and not filed electronically. That’s always fun. It’s good when the plan has the records showing the distribution but sometimes that’s 30 years old and records may or may not still be around.
Then there is M&A and Plan terminations where lost missing participants are always fun project.
Moral of the story, when you leave a company, consolidate you retirement account into an IRA as soon as you can when you can.