DWP investigation assistance sought.

If you get no knowledgeable responses I'd write and ask them if I wanted to know.

i'm guessing it means not from their own enquiries or staff, nor from other govt depts., local authorities, or enforcement agencies; so probably a member of the public.

Not necessarily. Could have been a random compliance check though these are quite rare. It depends on what they were investigating you for.
It must have been extremely distressing for you.

Do a subject access request.

It was probably a snitch.

I once made a SAR to an organization following a dispute and found a letter from a snitch dated many years previously. The name and address were redacted but I recognised the writing. Im no longer in contact with the snitch but if I ever ran across her I would challenge her with it.

I loath snitches. They are weak spineless people who hide behind anonymity.

I agree that it was either a random check or more likely someone reported you. Glad you can put it behind you as you won .

Thanks everyone for responding.