Whatever happened to this aircraft, I would suggest happened very quickly and was quite dramatic due to the fact it just "disappeared" in an area of good radar cover at 2300ft altitude, and there was no mayday, pan call or informative communication. IF icing was a factor it's possible he flew into or descended into an area of freezing rain which accumulates nasty clear icing very quickly. An aircraft with that low speed and very limited anti/de-icing capability would be in big trouble. IIRC freezing rain brought down a Citation (Mustang?) on approach into Friedrichshafen a year or two ago.