DWP investigation assistance sought.

Thanks everyone for responding.

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 .

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.

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.

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.

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