Replying to myself as I can't edit it any more: Apparently it wasn't a fire alarm from google but "Brunata Metrona Funksystem Star" [1]. It is still wirelessly connected though, not to the internet but to a ground station in the house, outside of the flats of the renters. Then once per year the station is read out by someone who connects a cable to it.

[1]: http://www.justiz.nrw.de/nrwe/ag_koeln/j2015/220_C_482_14_Ur...