Your guess is as good as ours. I have been told the give out a different telephone number than the helpline, but I have not seen this personally. In my opinion, they are a team that has been set up to verify details on renewals. By law HMRC have to carry out random checks on numerous claims and claimant types, to ensure they are doing everything possible to combat fraud. Have you had any contact with this team personally ??
My understanding is this team has been set up to give more specialist help to certain claimants - i think those who have been identified as possibly having more complex circumstances, those who may have missed renewals in the past and those who have fairly high awards where the risk of getting something wrong means the overpayment risk is high.
They just complete the renewal as the helpline do but this is more proactive to ensure everyone renews.