So a few questions:
- Am I obliged to identify myself to a police officer?
- Was the police officer correct in saying he didn't need a reason to search me or my belongings?
- I gather some of this may be related to having been in a 'border zone' at the time. It seemed in the situation that there's nothing I could've done to stop any of it; I could've asked for his details, I suppose, but that wouldn't stand in the way of a search. Do I actually have any rights in a 'border zone'?
Yes, yes, and technically yes but in reality mostly no.
1. Yes. You can be asked by police to identify yourself anywhere any time without cause.
You don't have to have any ID on you (althought that would be sensible and prudent), but as a citizen you are required to own such a national ID.
Typically virtually everybody (except women with ridiculously small purses) has their ID on them most of the time.
2. Yes. You can be searched without cause virtually anywhere.
3. It used to make a difference. Outside a "border zone" they'd technically need cause. But in the 90s most German states have declared their entire territory a "border zone".
Generally we don't mind this. Having the police search people they encounter in public space for contraband seems reasonable to most people. The concept of "cause" seems unproductive. Police will just make up some such cause and proceed to search you. It's just a waste of everybody's time.
Generally police will rarely make use of this privilege without being pretty sure that it'll be a hit. Actually being on the border would be the exception: There they somewhat randomly search people (young males mostly) in a somewhat drag-net-ish fashion without actually suspecting every individual client.
This is in somewhat stark contrast to other "German" notions about privacy: It's arguably harder than in the Anglosphere to search someone's home; police typically don't arrest most criminals longer than needed to secure identification and evidence; and of course we all royally wigged out over google street view (let alone unthinkabilities such as police posting mugshots on the internet).
The lived reality of all this differs of course quite a bit regionally, with Bavaria being the Texas/Queensland of Germany that it is coming down on the authoritarian (and stupid) side of things as it does in regard to virtually everything else.
I have no clue. Is border zone even a legal concept?
See above: Yes it is/was. It's called Zollgrenzbezirk.
By now it's obviously a somewhat irrelevant concept.