MSE News: Over 1m searches have been made on free online pensions tracing service

If you don't know about past employers with defined benefit (like final salary) pensions or pensions from contracting out, the HMRC contracted out helpline can tell you which place was getting contracted out rebates and when you were contracted out for each year.

Hopefully the pensions dashboard will become compulsory for all schemes, even if with no benefit details at all, to give a clue that there's one for you out there. Doesn't have to be a permanent live at all times system for this much functionality, just initial data loading and updates as needed occasionally.