I think the situation you mention has a significant distinction: Fraunhofer (or whoever) would presumably advertise that they expect you to get a patent license

I think the situation you mention has a significant distinction: Fraunhofer (or whoever) would presumably advertise that they expect you to get a patent license to use the code in a product. Either they'd explicitly state that up front, or their lawyers would say it with the first politely worded letter to someone who starts using the "open source" code. Software distributed under those terms would no longer adhere to OSI's definition of open source (I think it violates points 1, 3, and 6) nor the FSF's Free Software Definition (points 2 and 3). It might not stand up in court, but I do think it's obviously unethical to release patent-encumbered "FOSS" without mentioning your patents and then demand users obtain a license after the fact.