Absolutely not.
Thats kind of a chicken or egg question, no? The sponsor is required to provide statement, notices, and disclosures to the participant. The fact that employee never received anything doesn't mean that they were never a participant, it could be a failure by the sponsor to provide them with statements and notices. I don't think that you can require the employee to produce a document that the employer was required to provide to the employee, and then treat the lack of such document on the part of the employee as an indication that he/she was never a participant. I think that the the SSA notice is enough to put the document or research burden on the plan, not the participant. If the sponsor can't put their hands on those documents, its on them to try harder and spend more in order to get an answer. Again, Im not saying pay out, Im saying we keep sloppy records is not an answer.
I think most of them are people who got paid out but never removed from the SSA list as a code D, rather than employees who were never entitled to a benefit at all.
An answer like we went through our records and you were never vested or you were paid in 19XX is sufficient, but "prove it!" isnt.