Is it worth paying a service for unclaimed 401k's?

You're looking for one and considering paying a service?

Or you got a letter/call/etc saying that they've found something?

Maybe

Depends on how much is in the 401K it's up to you to pay the service fees. Why let them have your money?

Plan admins are required by law to report all retirement benenfits to the SSA. So when you apply for Social Security benefits, you get a notice of private retirement benefits due you.