What were four objects seen in Flightradar24 over Germany with the callsign TWR?
Many towers in Germany are equipped with FLARM so that they are electronically visible to FLARM equipped aircraft in the area. This is especially useful for helicopter emergency medical services and law enforcement that operate in urban areas during marginal weather conditions.
The fact that the towers are not always visible on FR24 may have to do with the fact that not all their receivers are equipped with FLARM. If nobody nearby is providing FLARM data to FR24, the towers are not visible on FR24.
The speed shown has to do with the GPS receiver in the FLARM unit; they sometimes detect small speeds, even though the receiver is stationary.