You are asking at least two different questions. As to whether you could be Jewish and "get away with it", at least until someone denounced you, I don't think it worked like that. Your ethnicity was known to the state, via your birth certificate and other essential documents. There were borderline cases involving people who were fractionally Jewish, and there were rules laid down for those. In the event of a dispute, I suppose your case went to judicial review/appeal. See also the Ahnenpass.
Your other question involves the status of German Jews in Germany around 1943. No the authorities didn't imprison or expel all Jews even as late as 1943. Your status and treatment depended on a number of tests. Far and away the best guarantee of your "safety" was to have a non-Jewish spouse. Also helpful was to have served in the German Army in WW1. I imagine financial means and influential connections also came into it.
So a Jew who could tick some of those boxes could still be technically "at liberty" in Germany in 1945 (as the diarist Victor Klemperer was). At liberty, in Victor Klemperer's case, meant being confined to a "Jew House" (extremely cramped communal housing), very reduced rations compared to other Germans, unable to work (except for humiliating work duties such as road sweeping and snow clearing which he was required to perform), and constantly in fear of arrest. But not necessarily actually imprisoned.
Being denounced to the gestapo need have nothing to do with being Jewish. Favourites included black marketeering, not complying with blackouts, listening to foreign radio, defeatist talk. Could it be your character was denounced for one of these "crimes"? If you faced one of these charges and were also Jewish, then you faced far graver consequences of course.
Then there were all the things that Jews weren't allowed to own or use (e.g. bicycles 1936, radios 1939). Your concentration camp survivor could have been denounced, not for being a Jew per se, but for flouting one of those anti-Jewish laws.