Something has just happened which I have NEVER seen happen before in my life

Our bus was stopped by the German border police (assisted by a single Polish border police officer) while we were leaving Germany and entering Poland

I showed them my Polish ID and they just accepted it, but they took the passport of every non-Polish national for a longer inspection, including my partner who is an EU citizen. She showed her Polish EU residence registration ID (which effectively functions like a national ID and should suffice), and they asked her for her passport anyway (which she also had on her just in case) and took it  

Then they sat inside a german police car looking through the documents for about 10 minutes, then they came back to ask for more documents from somebody, then after 15 minutes they gave back the docs to everyone

This was so bizarre, never had I seen any border checks at all when entering Poland within Schengen Zone, let alone this