proof of immigration status request to EEA national claimant

One of my client recently received a letter from DWP asking him for evidence of his existing immigration status.  He was asked for a copy of his Biometric Residence Permit or his passport with visa stamp/vignette.  If he cannnot provide these, he was asked to contact UKBA DWP status validation.
Has anyone else come across this kind of letter?  I just wondered whether my client (who is born outside EEA) was sent this by mistake or whether there is something more sinister going on?  My client would not have the right to reside under current rules but has been getting benefits since 2001 and has TP.