At the end of WWII parts of the German state archives were captured by the Soviet forces and taken to the Soviet Union. There they were placed in various Soviet state archives and kept out of reach of researchers. With the fall of the Soviet Union these archives were opened to researchers but not many people have taken advantage of that. Thankfully some of these German documents have recently become available online. The website of The Russian-German project to digitize German documents in the archives of the Russian Federation has uploaded a large number of German documents from WWI and WWII.
The site says:
‘As a result of the anti-Hitler coalition victory in the Second World War, documents of Nazi Germany turned up in many countries, including Russia. Largest collections of German documents are kept in the Federal archives of the Russian Federation (State Archive of the Russian Federation (GARF), the Russian State Military Archive (RGVA) and the Russian State Archive of Socio-Political History (RGASPI)), and in the Central Archive of the Russian Ministry of Defense (TsAMO). The project to digitize German documents was initiated by the administration of the Russian President in 2011. It is executed by the Russian Historical Society, the Ministry of Defense and the Federal archival agency with support from the German Historical Institute in Moscow. Coordination committee, overseeing the digitization project, is headed by S.E. Naryshkin, the Chairman of the State Duma of the Federal Assembly of the Russian Federation.’
Regarding German sigint activities during WWII, the search terms ‘Nachrichten Aufklärung’ and ‘horchtruppen’ bring up many interesting documents. For example:
Reports of Kommandeur der Horchtruppen Ost
Reports of Nachrichten Aufklärung Auswertestelle 2