I've been meaning to post about this ever since I started this fan theories community. Now that the rick nineg YouTube channel has recently posted a video about this unanswered question it seems like a good time to post my proposal for a solution.
This video is about Col. Klink (Werner Klemperer) possibly being Nimrod and the YouTuber plans on making other videos examining other characters as possible candidates for who Nimrod could have been.
As the video states there is no clue that excludes the possibility of Nimrod being a woman. If Nimrod was female then who are the possible suspects?
Since Nimrod is heavily hinted at having been present at the Stalag 13 POW camp during "The Missing Klink" episode (due to the note signed Nimrod) there are several main possibilities.
Since Gen. Burkhalter (Leon Askin) is in the episode it is possible his sister Gertrude Linkmeyer (Kathleen Freeman) might have been present off-screen. This would have given her access to Klink's office at some point during the events of the episode. She has been seen in five episodes of the series so she wouldn't have been a stranger to the prison camp guards. The problem with this possibility is that she lives with her mother and daughter so the opportunities to intercept information going to and from her brother on a regular basis about German activities would be rather limited.
The next stronger possibility is Klink's secretary. Klink had two secretaries, Hilda (Sigrid Valdis) and Helga (Cynthia Lynn), and Hilda is only seen in "The Missing Klink" episode. According to the Hogan's Heroes timeline "The Missing Klink" events occur around October 5-7 of 1944. Junior secretary Helga stops working at Stalag 13 around late May to early June of 1944. Why this timeline matters is that Helga was shown to be both knowledgeable and partly involved in the activities of the POW operations at the camp throughout her appearances in the series. Hilda was presented as more unaware of the activities but also willing to help the camp's POWs.
It's possible that Helga was visiting Stalag 13 off-screen during "The Missing Klink" but if she was Nimrod why would she have left such an advantageous position while acting as a successful spy for Britain? Then again, Hilda states she was "ordered" to be Klink's secretary after Helga departed. Maybe it was an order from the underground because Helga was starting to be suspected?
O.K. Now there is a third possibility that I came up with that I like as a solution to the identity of Nimrod. The caveat to my theory is that foreign non-English broadcasts that differ from English broadcasts are also canon.
According to an article on the MeTV network website and Wikipedia when Hogan's Heroes started being shown in Germany the dialogue was rewritten and Col. Klink was given an off-screen mistress/cleaning lady named Kalinke.
Canonicity can be argued but Kalinke being Nimrod might work well. Unfortunately, I don't know the German language and I don't know what part, if any, Kalinke has in the German version of "The Missing Klink" episode. Since the original writers never committed to a specific character being Nimrod it leaves all the original characters untainted while reasonably fulfilling all the requirements a secret agent would need to be Nimrod.