That one website that finds a lost 401(k) or pension from an old job

The Labor Department's Retirement Savings Lost and Found searches private-employer pension and 401(k)-type plans linked to your Social Security number and gives you the plan administrator's contact details. It is free, but you must verify your identity with a Login.gov account first (ID photos and a phone), which is why it needs signup; it does not cover IRAs or government plans.