If you left a job with less than about $7,000 in the 401(k) and never gave instructions, there is a good chance your money is no longer in the plan at all. Federal rules let plans force out small balances: under roughly $1,000 they can simply mail a check, and between $1,000 and $7,000 they roll the money into a "safe harbor IRA" at a custodian the employer picked, without your signature.
Why these accounts quietly shrink
Safe harbor IRAs default to capital-preservation investments, essentially cash, and typically charge account fees. A small balance earning almost nothing while paying $25-50 a year in fees erodes; over a decade the fees can eat a meaningful share of the account. That is the strongest reason to hunt these down sooner rather than someday.
How to find yours
The custodian was chosen by the plan, so the trail starts with the old plan: use the employer lookup to open the plan's page, then contact the recordkeeper listed there (the by-year history matters if the plan switched providers around when you left) and ask where force-outs from your era were sent. Also search your name at state unclaimed property and watch old mail for statements from unfamiliar IRA custodians: Millennium Trust (now Inspira), PenChecks, and similar names are the tell.
What to do once you find it
Roll it into your current employer's plan or an IRA you chose, where it can actually be invested. Compare costs first: our fee lookup shows what your current plan charges against its peers. If the old employer closed entirely, see what happens to a 401(k) when a company closes.