F1technical.net

http://www.autosport.com/news/report.php/id/104750

But the Belgian, who tested a DTM Mercedes last week, will only weigh up his options elsewhere if he is ruled out of a grand prix drive.

I've searched for this test, but I haven't found anything about it.
Anyone who knows where I can find more info on that test?

http://www.autobild.de/artikel/dtm-2012-3742921.html (german)

Apparently he was one of a number of drivers (amongst them: Turvey, Senna, Frijns) testing for Mercedes. Mercedes still has at least on seat to fill for next season, since Audi recently poached Green from them.