See message 3 here - one adviser in Middlesborough has been told that some fraud letters are randomly generated. I've no idea how true this is, but sadly I find it entirely plausible that someone at the DWP would think this a good idea. (I think I'm getting cynical.)
If a claimant receives one of these letters, I would seek an appointment with a solicitor immediately, if you can still afford one that is!
Any letters that suggest alleged fraudulent activity are a serious matter. If the DWP are actively involved in the issue of sham letters which I very much doubt, not only is it a criminal offence, but the DWP will need to substantiate their evidence before a court of law if they wish to pursue a suspected fraud investigation. If the allegations prove to be malicious and/or vexatious, they may very well see themselves in the dock!
Nothing on this board constitutes legal advice - always consult a professional about specific problems