Netzgesellschaft Entstörungsdienst: It's an emergency service for gas leaks[1] - it may feel implausible but it's totally common in Germany to have a "Netzgesellschaft" or some other very generic name like "Wasserverband" (water organization).
1: https://www.nbb-netzgesellschaft.de/ueber-die-nbb/entstoerun...
Don’t get me wrong, when I was in a tiny quiet village in the UK I once came back from the local grocery shop to find my front door had bounced open instead of locking itself when I’d left and absolutely nothing happened as a result of this mistake, and I don’t expect the same here; and sure, Berlin has a lot of graffiti, but I don’t feel fundamentally unsafe in even the most loudly afearing places like Görlitzer Park or Alexanderplatz — I don’t even get why the latter is on the list of places people talk about when suggesting danger.
Really, the worst I experience here is the fire brigade and ambulance sirens (and, confusingly, one time where the sirens were on a van marked “Netzgesellschaft entstörungsdienst”, which both Google and my own limited German think is something close to “Network company anti-jamming service”, which feels implausible).